Relationship advice?

I've seen others go through this, but this is my first time dealing with it. The guy I'm with now was in a long term relationship that just wasn't good for either party, and as soon as we met we hit it off. He broke up with his gf, and now we're dating.
But it's really weird for me because the relationships had some overlap, and while they broke up, things are still messy and there's still feelings on both sides of that. I hate seeing him hurt over her, but I'd be crushed if they got back together.
What do I do???