how do i get my man to leave the past in the past?
You cannot make another person do this, this is a personal choice to let go of the past, he has to realise it on his own. The more you push the issue the more he;ll hang onto it. He has to get over it in his own time, all you can do is be there and support him.
As above. Talking about it can help. If he can't talk to you then maybe councelling.
Do you mean a past love, or everything in general?