Everyone finds it hard to let go of someone they still love desperately.
But some people don't just hang on to exes, they're still clinging to the expectation they'll be reunited, years after their ex has moved on, married or even had children with someone else.
Remaining fixated on someone who rejected you long ago is, at the very least, unhealthy psychologically.
At its worst, it can tip the sanest person over into Fatal Attraction bunny boiling territory.
How do you tell the difference between mourning a lost love and becoming a delusional fantasist?
Here's seven signs you're well and truly stuck - and how to move forward.
1. IT'S LIKE THEY NEVER LEFT
The wallpaper on your phone and computer screen is still a photo of the two of you, you talk about them all the time even though it's been years since you saw them, every day or week marks an 'anniversary' of something you did together in the past.
"Three years today," you say wistfully, "John and I were in Thailand on holiday."
2. MEMORIES ARE ON A CONSTANT LOOP IN YOUR HEAD
If you're constantly playing memories and past conversations or fantasising about what you'd say or do if you meet your ex again, you're 'ruminating'.
Ruminating means repeatedly focusing and thinking about a situation, it's causes and consequences.
Circular thinking is another sign you're obsessed: you'll allow yourself to be led away from the idea that you and your ex will reunite but just when the person trying to talk sense into you thinks they've got somewhere, you go back to square one with "Well that's all well and good but I still know it's not over".
3. YOU'RE CONVINCED THEY'RE STALKING YOU
It's not unusual for people hung up on an ex to try to turn it around and pretend it's their ex that's hung up on them.
"How come his car is always parked near mine?", one woman I counselled would say about an ex of hers she broke up with four years earlier.
"Because he lives and works in the same area as you?", would be my logical answer - but she wasn't having it.
She spent her life making sure she was in places where he would be and when he did turn up, she'd phone her friends to complain about him 'stalking her' and 'not letting her move on'.
Any new relationship that didn't work was because he's somehow 'got to them' to warn them off her.
"He doesn't want anyone else to have me," she'd say.
In reality, her ex moved on rapidly and was already living with someone else six months after the split. Even this didn't deter her.
The new woman had 'moved herself in' and she could see he 'wasn't happy about it'.
The facts didn't matter: she had no intention of letting go of the idea that he couldn't let go of her. Some people never do let go.
A colleague of mine has been married three times and has two children and still mourns the 'love of his life' that he dated at university.
"Not a day goes by that I don't think of her," he says.
4. YOU OBSESSIVELY STALK ON SOCIAL MEDIA
The most common and obvious way to stalk an ex is through social media.
Even if they're savvy enough to make their accounts private, trawl through all their friend's Instagram, Snapchat, Facebook or Twitter accounts for long enough and something will turn up.
If they look happy in the photo you've found, it's like a dagger plunged through the heart then twisted.
If they look unhappy, it's worse - you convince yourself they're missing you as much as you're missing them.
If you have passwords they've neglected to change, you might be checking their personal emails as well.
5. YOU HAUNT YOUR OLD HAUNTS
If you're deliberately going to places where there's a high possibility of running into an ex partner or taking detours to go past their house or work place, you're stuck.
Ditto hanging out with friends who know your ex well purely to fish for any morsel of information you can get.
6. YOU'RE STILL FRIENDS EVEN THOUGH YOU'RE SINGLE AND THEY'RE WITH SOMEONE ELSE
On the surface, this looks very adult and mature.
You've split up but still love each other as friends, why should you throw away all those years together?
But let's be honest here: there aren't too many people who've been in intense, long-term, committed relationships who can move easily and immediately into friendship.
The dumped person's motivations are questionable.
Is that 'friendship' really as innocent as you make out? Are you sure you aren't seeing them in the hope of catching them at a vulnerable moment, hoping they will suddenly realise they should never have let you go?
7. EVERY PERSON YOU DATE IS COMPARED TO THEM
Needless to say, they never measure up.
The longer you've been separated, the more likely it is you've idealised the relationship to the point where no-one can possibly compete.
Not even your ex could live up to this glossy, edited version of them!
You answered 'yes, yes, yes' to all of the above?
The first thing to do is to accept there's a problem. Stop kidding yourself you're doing just fine: you're not. But you will be.....
HOW TO MOVE FORWARD TO A HAPPIER FUTURE
Here's some practical ways to move forward fast.
1. GET SOME CLOSURE
If you're not quite sure what caused the break-up, you can't help but keep dwelling on it. Apart from not being able to learn from your mistakes, curiosity alone can drive you crazy.
If you're the sort that can handle feeling vulnerable, contact your ex (if you know where to find them). Explain you're having trouble getting over them because you don't have a clue why you split.
Ask them to tell you, straight out, without sparing your feelings, what it was that caused them to leave.
Take it on the chin, analyse it for a week, then drop it. If you can't, consider counselling.
2. CONSIDER COUNSELLING
If you can't drop it, consider counselling. If you're too embarrassed to do this or your ex moved countries (and funnily enough, didn't leave a forwarding address), ask a close friend who you trust to give you their opinion, without sparing your feelings.
If they won't or can't help you, again, book a few sessions with a good therapist. They will help you make sense of it all.
3. GIVE IT ONE LAST TRY
The break-up was your fault and you can't stop thinking, "If only I'd done this or that", again, call your ex and ask for another chance.
Explain the reason why it didn't work last time for you and your reasons for thinking it might now. Even if they knock you back, you've given it your best shot now and that's all you can do.
4. REMOVE ALL REMINDERS
Not just photographs or things around your home that strongly evoke memories, if you can't move forward and it's been years, it's worth changing jobs, moving house, even changing cities if you have to.
Drastic measures, I agree, but necessary if it finally gets you over them and into a fresh, new life.
5. BREAK ALL TIES
Don't see them, don't be friends on social media and if you have mutual friends, don't pump them for information.
If you can't cut all links (you own a business together or have children), keep contact minimal and conversations purely functional.
6. KEEP A 'STALKING' DIARY
You know it's ridiculous but you find yourself parked outside their flat, watching the window - again.
You've just gone six tube stops in your lunch hour to watch them eat a sandwich in the park.
If this is you, start a 'stalking diary' and write in it before you're tempted to play Sherlock and when you get back again.
One of three things will have happened after you've stalked them.
The first is you've seen something that's upset you (that is, they've found someone else).
Equally likely: they caught you and you feel like a complete idiot, or nothing happened and you just feel empty and lonely.
Record all these feelings and when you're tempted to stalk again, use the old smoker's trick of putting it off for five minutes.
Spend that five minutes reading about how awful you felt after the last episode.
7. REMEMBER YOU HAVEN'T LOST 'THE LOVE OF YOUR LIFE'. YOU'VE LOST ONE LOVE OF YOUR LIFE
Many people can make us happy, not just one person.
It doesn't mean you'll never be that happy again if you don't reunite with your ex.
You could end up even happier than you were and love the new person even more than the last.
You certainly haven't lost 'the one chance you had at happiness'.
8. WORK ON YOUR SELF-ESTEEM
People sometimes remain stuck on their exes because they secretly worry the rejection was deeply personal: in short, that they're unlovable.
They hang onto the memory of the ex because at least that was proof that someone did love them at some stage.
If this is you, take control of your life.
Ditch any friends who bring you down, see more of friends who lift you up.
If you hate your job, find a new one. If you hate where you live, find a new place to live.
Make a list of 10 ways you'd like to improve your life and tackle one thing a month.
9. ACCEPT YOU MIGHT NEVER FEEL GREAT ABOUT THE SPLIT
It doesn't mean you're still in love with an ex if it still hurts when you think about them.
It doesn't mean you're not over them if you still miss them now and then.
You're human: you loved them, you thought you had a future with them, it's human to grieve broken dreams.
As a very wise friend said to me recently about a breakup, 'It's not the past memories that get to me, it's the future memories. The fact that we aren't going to have the life I thought we were going to.'
10. GET OUT THERE
Give yourself time to grieve (and there is no set rule of how long it should take though it takes an average of 18 months to get over a marriage break up) but then force yourself back out in the world.
Don't try dating if you're still feeling raw, but do put yourself in situations in where you'll meet new people.
Distraction is a highly effective way to move forward.
If the pain is still intense and unbearable despite your best efforts to move on, give up fighting the battle on your own and get some therapy.
The right therapist will help you work through the reasons why you don't want to let go: there may be other issues beneath the fixation.