Whats my sexual orientation? is it a phase?

Okay so ive been struggling with my sexuality for about 2 years now. I basically have been frigid towards boys my whole life and i didn't really understand why i could bring myself to kiss them, be in a relationship with them, or have an romantic actions with them. But i did find boys physically attractive and have crushes on them, actually in middle school i was probably "boy crazy" (but for me it was basically thinking boys where attractive only because i still didn't date them) until i grew out of it in about eighth grade. Anyway since about seventh grade i have been physically attracted to girls. At first i didn't realize it but then i noticed that when a cute girl walked by i would usually take a second look and check them out. But i thought that it didn't mean anything because i identified as straight, but then i actually thought about it and noticed that i actually thought about sex with girl during masturbating and enjoyed. So i put these feelings off as purely sexual until three months ago when i noticed this butch girl at my school. She is absolutely beautiful in my personal opinion, and this is the first time that i have actually want to be in relationship with someone and that person is her. I would kiss her and be romantic with her. This has caused lots of thought going on and i have thought about marrying a girls only and only having relationships with them in the future. These thoughts are much more than i have ever experienced about guys and ever since i have started liking the butch at my school i am not interested in boys anymore. A lot of people are telling me this is a phase but i really don't think it is. I feel complete. Btw im almost 15 and im a girl. How do i know if its not a phase? And what is my sexual orientation?