GIRLS ONLY. need advice?

well i liked this girl since 7th grade. i went to the movies with this kid and all these girls showed up and they were gonna be supposidly all these "hot" girls there. the ones that were supposidly hot, weren't. all i kid see was this one girl. i dont wanna sound like mooshy but she literally glowed. i was so excited because we sat next to eachother and some girl next to me was a little flirty but i ignored her and paid attention to the girl i wanted. i made her laugh a lot and we just met and we were so "corny" because we made a big deal about touching pinkies. i asked her out a few days after she said yes i was so happy it seeemed like it would work but idk i just interest. i txted her a few months later after i had realized her i needed her more than anyone. it took a few months to get her abck but i did even though she had two bf's in the process. they were doosches. it was novemeber 28th when i asked her out again she said yes. i was soo ****** happy. we went out for two years and recently broke up in oct. we went through so much and made a promise to never break up through highschool. since the break up weve faught a lot. she wrote a letter telling me she broke up with me because she wanted to live the high school expreience. i was devistated because it seemed like she wanted to get away from that. i wanted her to hang out with this girl she was friends with so i didnt see her on some weekends but i was ok because i knew she was developing a good gf relationship. i think it was a mistake now because that girl is the one who took my girl away :( i know it seems far fetched but its true the girl always parties, and said she didnt. the friend also hooked up with some 10 guys at new years. i was home alone (friends ditched). and my ex hooked up with some guy she didnt know. :( the letter also said she thought i didnt love her because i wouldnt listen. i was trying to change but she couldnt see. we got in a fight because she ditched to go to some dinner. she left me after that. as i said we have faught a lot. and her friend has gotten involved. which she ****** up things even more again. i miss her a lot because she was the one who i could tell everything and did. we went through so much and did so much. how coem i still love her after what heartbreak shes given me? and why does she do it to me, the one person who reallly cares. just shows it differently but she understands it.