I have a serious problem.
I'm in a relationship, first one since my last big breakup. But remember that ex that was part of the big relationship? I miss him so much. Him and my boyfriend are a lot alike. It's not on purpose you know? I just like guys like that. I'm happy with my boyfriend. He really does make me super happy. But I still miss that ex, and a part of me still thinks I might still love him. He claims he still loves me and that he's sorry for doing this to me and all that. He's still with the girl he cheated on me with and he's engaged to her. But he says that they're not what they used to be, so he claims. I'm not necessarily sure I believe him.
and now, I think I might be fallin' for the guy, I'm with. But I have serious trust issues now and Idk how to get rid of my ex from my mind..
Look, it's understandable that you'd be nostalgic about a previous relationship. It happens to everyone, I think. Having him piggy-backing off of you wouldn't help the feelings, either. The fact that he hurt you to the point where you now have trust issues also makes it more understandable that you'd be feeling how you are now; you're feeling things for your new partner, but you can't help but think of what would've happened had he not hurt you - i.e., would you still be with him? Would you have trust issues? etc. The issue that he's now apologising for what happened, and claiming that you are better than his current fiance, seems to propound the feelings that you're having.
It is completely understandable, and there's no real way of avoiding it. Since he has already apologised and you seem to have accepted it (at least, in part), then it will be on your mind for a while. The only thing you can really do now is break contact with your ex, especially if you're feeling things for your current partner and don't want to screw up. You'll miss him for a while, but you'll soon forget about him.