Friendzone/breakup advice needed.

Okay so this is kind of a long story so bare with me.
I'm a 15 year old Christian and I had liked this girl who is the younger sister of my best friend for several months and eventually I got the guts to give her this letter (yes I couldn't talk to her cause I was too nervous) which explained how I felt and everything. Then, a couple days later, her parents gave me a ride to Six Flags since my parents weren't available and basically her Dad told me he read my letter and he wanted me to know that I had permission to date her once I was older but for now we could just hang out and such even if we liked each other. Now nothing really happened for a couple months because I hardly ever saw her and when I did I had my friends and she had hers and at our church all the boys and girls are extremely segregated so it's nearly impossible to talk to a girl since you've got all your guy friends on your butt. But right before I left for vacation last November I got her number and we ended up texting super late every night during my whole vacation and continued that for several weeks after I got back. We both learned a lot about each other and grew pretty close and it seemed like everything was great and the next couple years were set up for us to start dating once I was driving. Now, a couple facts about me. Idk if these are really important but it may be. I'm pretty tall and handsome (according to other ppl) and I'm very socially awkward in person and not very approachable if you see me across the parking lot. At home I have some parent issues and a quick temper and some anger issues. So, around Easter, she texts me with "hey I need to tell you something so get ready" and about an hour later she sends me a text explaining how because of our very few differences and the fact that we barely talk in person, we should just be friends. Now me being me, I instantly started trying to get answers because this was completely random. I'm very close to her family since I've known them for several years, their son is my best friend and her parents are some great ppl who are okay with me and her being a thing so this was shocking to me. For the next 3 hours I argue with her about why she's doing this because her first reason gave no actual reasoning. Eventually, she comes out with "I don't wanna potentially date someone who has anger issues since I don't unless someone else is upset". Now since this was the first actual answer in 4 hours I was satisfied UNTIL I got to sleep on it and realize the bullcrap in that reason. When I text her back I find out that she is going to visit her grandparents for 2 weeks and so I won't have any answer till she gets back which Gives me even more time to think. My logic is that if she didn't wanna start anything because of my anger issues, why wouldn't she take into account the fact that I've still got several years to mature and grow out of it because we'd even agreed that we wouldn't date till I could drive which won't be till next summer. Btw, she is a year younger than me. So once she gets back and replies to my text I demand the actual answer and she tells me (breakdown version) I don't wanna get your hopes up now in case you're some jerk when we're dating because then it'll be that much worse when I break up with you so I'm ending things now before anything bad happens. So I pretty much quit asking and haven't texted her since around April 1. I'll still see her around 1-2 times a week but avoid eye contact or any involvement with her at all. And when I see pictures of her happy it makes me angry for some reason. Now my question is what happened? I didn't leave anything out and I have no idea why this happened. Is there something I can do to win her back? I still like her and I found out from her brother that she still likes me too. I really wanna fix whatever went wrong because I feel like this stuff could really be fixed and one day I can date her for real but right now I need advice. Thanks.