Dwelling on the past may not only stop you from enjoying each day to the full - it could also be bad for your health.
Research suggests that people who look back at their past experiences full of regrets about missed opportunities or with bitterness about how they have been treated are more likely to fall ill and generally have a poorer quality of life.
Those who look back in anger are also more sensitive to pain, it found.
It also suggested that focusing too much on the future does not harm health - but can stop people enjoying what they have.
The happiest and healthiest people, according to the researchers, are those who manage to enjoy the here and now, while making time to learn from the past and plan for the future.
In the study, 50 men and women were asked about their feelings about the past and future, as well as their physical and mental health and quality of life.
The questions included how often they think about things they should have done differently, whether they worry about not getting things done on time and whether they live life a day at a time.
An analysis of the answers revealed that those who dwelt on the bad things that had happened to them tended to be in worse health.
University of Granada researcher and co-author of the study Cristian Oyanadel said: "According to what we have observed, the most influencing dimension is the perception of the past.
"We have observed that when people are negative about past events in their life, they also have a pessimist or fatalistic attitude towards current events.
"This generates greater problems in their relationships and these people present worse quality of life indicators."
Explaining this, he said that such people find it hard to make a physical effort in their day-to-day activities, are more limited physically at work, more sensitive to pain and more likely to become ill.
"Furthermore, they generally tend to be depressive and anxious," he said.
Looking to the future is not necessarily bad for our health, the experts added - but quality of life suffers because such people tend not to enjoy what they have.
"People who are more future-focused - those who put their personal goals before everything - forget to live pleasant experiences and are not very connected to their positive past experiences.
"They are not physically or mentally unhealthy but have a lower quality of life than the well-balanced group."
The people who are best off are the sensible sorts who have a nostalgic view of the past and manage to learn from it, rather than let it drag them down. This means they plan for the future but do not neglect the present. Many previous studies have linked a person's outlook on life to their health. One of the most recent found hypochondriacs really may be destined for an early grave.
Those who complain about their health are up to three times more likely to die in the next 30 years than those who regard themselves as more robust, it suggested.
The findings, in the PLoS ONE journal, allowed for any subjects having heart disease or other serious illnesses at the start of the study, or being on medication.
A person's outlook can influence their lifespan. On that basis, the study suggested that doctors should not limit their definition of good health simply to a patient being free of the physical symptoms of disease.
It is thought optimists fare better because they refuse to let bad health get them down.
They may also be more likely to follow their doctor's advice, believe in the benefits of a good diet and exercise and be better at handling stress and its effects.
- DAILY MAIL