Nothing seems right

I've only asked one other question on here before. I needed outside help and now I'm back again because nothing ever seems right. My girlfriend and I have had problems throughout our whole relationship, always arguing and we were never able to solve our problems. We tried over and over and over, countless times, after almost two years together I decided that it was best for both of us if we just called it off and gave up on trying so hard if it wasn't going to go anywhere. I still love her and still want to be with her but I know its better for us not to be together. After we broke up I chose not to talk to her because it made it harder for me since I did still have feelings for her. She insisted on talking so we did and I told her I wanted to be friends, we tried it but it didn’t work out for her because all she wanted to talk about was us again. I'm surprisingly doing fine with the break up, whenever I don’t have contact with her I'm fine with it almost as if I wanted to break up, so I guess I kind of don’t want to be with her. We try to talk as friends, then take a break from it, and back and forth, it's just one big confusing mess. My mom loves this girl by the way, and is always backing her up and try's to find nothing wrong with whatever my ex does (by the way I'm 15). Since my mom and my ex are so fond of eachother, my mom invited her to the beach with us,and my ex said yes even though we still have problems together. That’s only half the problem, this makes it 2x worse. Like I said my girlfriend and I have had problems our whole relationship. I met this girl at a dance practice I started having for a party, the girl was made my partner by the dance teacher, after that this girl and I started getting close. I started liking this girl and she liked me too even though we both had a significant other. This girl had nothing to do with my break up with my ex, maybe she opened my eyes to finally give up or something but it wasn’t because of her, I wouldn’t give up 2 years of a relationship for a girl I just met. My ex knew this girl from school, which makes it worse (I don’t go to their school). I decided to tell my ex after I broke up with her that I was talking to this new girl, BIG MISTAKE, and she started thinking it was because of her that we broke up. The few people that know about me and this new girl all think it was because of her that I broke up with my ex and I see why they would think that, I basically broke up with my ex around the time I was getting close with this new girl (but remember that was not the reason, I guess it's kind of a coincidence). Anyways me and this girl started talking everyday and then started talking on the phone like every night till like 6am since we both stayed up late. I started liking her a lot, but she still had her boyfriend. We have talked about being together and everything like that but she still goes out with this guy, I'm guessing she doesn’t want to risk breaking up with him even though she thinks I'm like “the sweetest thing ever”. We have discussed all this and I have came to the conclusion she wont break up with him for me (even when we started talking I figured I wouldn’t have a chance with her so that adds more reason on why I wouldn't break up with my girlfriend for some new girl that I might not even have a chance with). I still have hopes for me and her though because everything just feels right with her and I love talking to her. I just don’t know what to do with it though. I like talking to her but it makes me want her more and I cant have her which makes it hard for me and I don’t want to stop talking to her because then it might ruin my chances with her if something does end up happening for me and her. I need advice on weather it really is better for me and my ex to be over and done with all our problems and unhappiness even though we love eachother or be together with the same problems and unhappiness. Then I don’t know if I should tell the new girl I don’t want to talk anymore because it hurts me to talk with such strong feelings but with nothing positive ahead between us, or keep talking and hoping that something happens but hurting while I wait for something to MAYBE happen for me and her.