I've known a lot of stressed-out married people who a lot of times believe the grass is greener on the single side of the fence, and vice versa, but the thing to remember is that despite any of these posts and articles of people's OPINIONS (though I have seen studies that suggest that women, particularly older women, are healthier and happier being single though can be opposite for men), there are plenty of people who are single by choice and they are very happy remaining that way.
Being in a relationship is a tremendous amount of work and it's even worse if it's not particularly a good match. Marriage in and of itself isn't a solution to unhappiness and can oftentimes make things worse depending on the underlying issues, but there are plenty of happy people in marriages too.
If you become a happy person, you will be happy regardless of whether you are single or married.