Idk what to think. My boyfriend makes me feel like there's some one else, he lies and I catch him up but I can't ever prove there's another girl involved... he's cheated on me before but says he's trying to change to make our relationship better.. I constantly accuse him of cheating and continue to fight with him about it, he says I'm pushing him away. I just want to forget about the past and move on but I don't know how to be sure he's being hones. Am I just insecure or is something really going on!

Once a cheater, always a cheater. If you can't handle it then you should have never gotten back with him in the first place.

I don't necessarily think you are insecure, I think you have had your trust in him broken.
What I do know is that if he has cheated once, then I would bet my paycheck he has, is, or will cheat again.

His lying doesn't do much for helping to rebuild the trust in the relationship. What does he lie about? What does he say when you find out he has lied?

As long as the lies continue, you won't be able to trust him. So discuss it with him. Find out why he lies. Does he know that what he is doing or hiding will make you upset? If so, and he does it anyway, that is a problem as well.

Decide whether he can be honest and you can stop looking to catch him. If it is no to either of those, or both, you will need to reconsider the relationship and decide if it is the type you want to be in.

How has he changed to try and make things better? You won't be able to forget about the past, but you can, with his help, work towards rebuilding trust for the future. But it can only happen with openness and honesty... and time. Without all of those, it simply won't happen.