Need for a 2 year relationship

[Albinoninja]

As the title says, I've been dating my girlfriend for a little under 2 years now (20 months to be exact) which is pretty long since we are both still pretty young (I'm 17, she's 18). Here's some background info: we were friends our freshmen year of high school, and started dating towards the beginning of sophomore year (so yes, you can get out of the friendzone). It was kind of like a movie start to a relationship because she liked me the whole time and me, being oblivious, didn't know, or accept it until she showed me how she really felt. At first I was hesitant, but then I though that I'll give it a try and soon enough we had a great relationship. I'm attracted to her physically and mentally (obviously she has flaws though, like us all) and I feel like I can say anything to her. We usually act how best friends act, teasing and pranking eachother, but we also have plenty of intimacy in our relationship. She has gone through hell, because her family kind of sucks, and I have always been there for her which made our relationship even stronger. Sounds like a great relationship right? But I wouldn't be here if nothing was wrong. Within the past week, my feelings for her have suddenly died. I still care for her and am attracted to her, but something feels off. Like I almost feel like we overdid it and now I'm bored. I feel terrible because it would crush her for me to tell her this (I'm all she's got) but I don't want to have to pretend. The only things I can think of that would cause this is that we show constant affection towards each other (almost too much) and although we only see each other once a week (besides school) we text constantly (of course since we are high schoolers). But how can I start losing intrest witin a few days? Nothing has happened in that time. I am confused and worried, because this feeling caught me by surprise. I don't know what to do.