She gushed: "I've never been more in love. I'm so excited to have him in my life as my best friend and my boyfriend."
And she also said she is "grateful" for the That 70s Show actor.
She said: "We don't share everything with the world. But we do feel comfortable to share a lot. With Wilmer and I and our relationship, he's extremely supportive in everything that I do. He makes me feel confident and beautiful. And I'm just really, really grateful for him."
Demi and Wilmer are so secure in their relationship that he didn't even mind when they bumped into her ex Joe Jonas and his brother Nick Jonas while on vacation.
Alongside a picture of the three guys, Demi wrote: "This picture makes me so happy. I love my boys."
And Wilmer, 35, also shared a picture of himself and his pals sitting on some steps. He captioned the image: "SharingTheMoment with woes in St. Barts or St. Barths....Tomato...TomAto (sic)"
Meanwhile, Lovato says she is "grateful to be in recovery".
The 23-year-old singer - who sought treatment in rehab for an eating disorder, self-harming, alcohol and drug abuse in 2010 - took to Instagram to share an emotional message as she encouraged people to face their addictions.
She wrote: "I am one of the 1 in 7 people who has faced addiction, and I am grateful to be in recovery today. It's time for us all to reflect, and start @facingaddiction. Let's stop ignoring the biggest problem in our country we often don't want to look at. We all are connected to this issue one way or another and we all need to be part of the solution. Together we can replace shame with support, hope, and healing. #FacingAddiction #selfie (sic)."
Demi is taking part in non-profit Facing Addiction's new campaign to raise awareness of addiction and she, along with a number of other celebrities, posted a video of a black screen with the words: "Look Close. What do you see?".
The campaign hopes that people will take a good look at their own reflection in their screens and the video ends with the words: "1 in 7 will see the face of addiction".
Demi previously admitted she is a much more emotional person since conquering her addictions.
She said: "When you are sober, you experience everything on a heightened level. If you are going through something stressful, you're extra stressed. I used to sit through a scary movie and not be scared once. At haunted houses, things would jump out at me, and I would sit there bored.
"Then I got sober and I feel everything. I cry more. I f***ing laugh harder. Everything. You have to think, if you are chasing the next party your whole life and you always have to be numb, is that something to be proud of?"
