Does my girlfriend really love me?

My girlfriend and I have been together 2 years and 3 months we are both juniors in highschool. We met our first class of freshman year. She was the first girl I ever crushed on liked or even had any feelings for. I noticed she and my best friend texted all the time. We all played Xbox live together and when I would get off they would stay on! Finally after a year I found out she cheated on me with him and she cried and told me she was sorry. I have her a second chance and she lied to me about a lot of petty things. finally after our 2 year anniversary things finally cleared up and we had the occasional fight. But now it's bad again. We had planned to go to the basketball game at our school. We planned to just stay at school. But after school she stood me up to go get 1 pair of jeans and a salad at zaxbys that she could have done at anytime. Me and my mom sat in dobyns bennett high school parking lot from 6 pm till 708 pm waiting on her to get there. I got yelled at a lot over it and when she finally got there I was pissed and Hungary because I didn't eat All day because I planned to eat at the game at 430 not 730. Well we didn't talk much of the last and only game we got to see and finally she started telling me that at least zack ( the boy she cheated with) was a descant guy and that he truely cared and I don't after I have put up with all that has happened between us! We have talked About marriage and both have said we want to get Mariried after highschool. And tonight she fought again and told me she didn't want to marry me and she meant it. All because I was telling her that I was worried about her going to DC to march in the inaugural parade And she told me to stop BITCHING!! I'm really getting sick of it and I think she is my true love!!! Please give advise to me! And I'm going to show her this!!! Thank you for reading this!