You deserve so much better than that, hun! You deserve someone who's going to focus on YOU and you ONLY.
I know you might be in love with him and it might be hard to let him go but you have to tell him that you're not a second choice! If he really loved you as much as he says he does, he wouldn't be able to have love for anyone else! It's either you or her, and it shouldn't even be up for debate.
I really think you should find someone better, dear. Someone that won't find someone else and lay that bullshit on you. Good luck with whatever you decide to do xx