one day we'll get nostalgic for disaster

I hear these two expressions frequently:

if you love him, let him go
if he comes back-his heart is yours

&&
you break up for a reason

How do you differentiate between the guy whose heart belongs to you ( the one you give a second chance, and take back) and the guy who was meant to stay in the past? They all say the same thing when they try to come back into your life-

:I'm sorry
:I made a huge mistake
:I will never going to do it again
:I want you back

Are they realizing this b/c you left a huge void when you two broke up? Or is it jealousy b/c you've moved on and he's lonely? How do you know ?
ugh
Are second chances even worth it ?
I cant help but think we throw ourselves back into these situations because we're in love with what could be. There's always an air of hope we have - that things will turn out better than they once did.