Why do I still want to be with her?

I dated this girl from 4 years, I really loved her, we had a lot in common, and we shared a lot of similar interests. About 6 months into our relationship she wanted me to take her to the pet store to get her guinea pig food, I didn't have much gas or money for gas at the time, so I said sure, could you help me with gas, she flipped on me and I didn't hear from her the rest of the night, the next day I found out the reason why she didn't text me back is cause she had her ex boyfriend take her instead and he took her to dinner and stuff, she came right out and told me this.We broke up for awhile and got back together things were good for about 6 months and then she started acting distant and wouldn't communicate with me and we would fight a lot, cause she'd act distant toward me and would snap on me alot, and I would get frustrated and say why are you always yelling at me and do you even want to be with me cause you seem like you're unhappy and stuff. So after months of this, she broke up with me and over our break up she was having sex with her ex. The same guy who took her to get her pet food and stuff, but we were broken up so I let it go. But when I took her back she treated me real bad, like said I had a small penis compared to her ex and said I sucked in bed. So I started to feel insecure and asked her if she love me and wanted to be with me and stuff, so she broke up with me and on the same day we broke up she had sex again with the same ex on same day