I should have clarified that it's most common in recently divorced men, but not exclusive to them. Any time a relationship into which you've become heavily emotionally invested ends, this feeling is likely to pop up.
As for a fix it, it's really up to you. Your brain is a bit like a grassy field. Paths that are regularly used are beaten down and easily traversed. In order to teach yourself to think differently, you have to do so constantly and consistently until the new path is beaten down, and the old path is overgrown.
Analogies aside, it's time for you to take some "you time" until you're genuinely okay with yourself, and the relationship statuses of those around you won't matter in the least. It's not a recent thing, actually, but lately I'm noticing more and more that people validate themselves based on their relationship statuses as opposed to their inherent values and virtues.
In a nutshell it boils down to how you feel about yourself.