Nope, ex's are ex's for good reason, I don't live in the past, life's to short.
Remind yourself of the bad times, the annoying little habits etc! Find things to fill your time so you don't have the time to sit and obsess about them.
Don't worry eventually you'll forget the exes and just be bitter
Cruel! But quite possibly true.
It sounds like you are lonely, and thinking back to times when you had someone close.
Keeping busy is good for a while....but often you need to learn to trust again slowly, and take the risk of starting to let someone special close. Just take it slow.
I've never dated and I have no trust.