After a difficult decision is made, we immediately doubt our choice. All the reasons why we should have chosen to do the opposite flood our thoughts. We blew it, and now it's too late.
At the same time, we somehow forget the many reasons we did have for making that decision. Once we're removed from difficulty and anxiety, it seems like it wasn't that bad. The reality is, if we backtracked into the situation we would become painfully aware of our reasons once again.
It almost seems like a no-win situation. Stress when you're in the situation, regret when you get out. The key is, learn from what's behind, but move on. Look forward to the next chapter. If you jumped into a novel halfway through, things wouldn't make sense. You wouldn't know the characters or plot. But read it from beginning to end, and you'll see the painting each stroke was creating.