Will my girlfriend ever choose to be with me?

I know this is long for some of you but I finally got a girlfriend but she later told me that she already had a boyfriend who doesn't go to our school. She also revealed that she is not a virgin and I just got back together with her after breaking up with her because I believed that it's okay to go out with her even she is not a virgin. We knew each other for a long time and she been in love with me for three years no matter how mean I was to her throughout those years until I was in love with her. But I really don't know that just because she really likes me doesn't mean that she will choose me because she was taken long before we met. I texted her very nice things last night and she said that what I said was sweet but she also said that she is mad at that boy who like her because he wasn't calling her at all yesterday. So we been a couple for three days now and I'm really scared that I might loose her. Especially she is my first girlfriend and Valentine's Day is in three weeks. I was hoping to have my first V-Day with my first girlfriend. I don't know what I should do so she would choose me over her old boyfriend. I really need help and fast, I don't know what to do, I don't want to suffer from depression ever again because of this. Any ideas or suggestions? I need as much answers as possible to figure this out.

P.S: We been texting a lot and I'm trying to find some quality time with her. We also made a lot of contact in school.