I don't think the question is how to move on from him; it seems that the real question ought to be how do I learn to be happy without him?

If this guy has a girlfriend but tells you that he still holds feelings for you while in another relationship and fools around without regard to the feelings of others, he doesn't sound like someone worth pursuing.

Heartbreak is hard and everyone goes through it; the important thing is to remember to love yourself first before ever loving another. And in that same respect, learning to let go of someone you love because its necessary.

Hope that was helpful!