How can i get out of this without destroying her?

Basically, I met a girl on the internet and we started talking, I found her really attractive, but she is one of those girls that thinks she's a 2 when she's more like an 8.. and constantly goes on about it. Sure enough though, she is 15 and I am 19, which for me isn't just illegal.. but tests my own morals and I find it very awkward (i mean i would even feel a little dodgey getting together with a 16 year old). I told her that I was uncomfortable with being anything more than friends and I thought she understood.
So we continued to talk and I didn't notice it then but I do now, she was getting more and more into me. She would moan about being ugly and fat.. and me being a little slow to be honest, would buy into it and try to make her feel better with compliments, which I don't think helped the situation.
I wasn't nearly as worried until the other day she said she was thinking about me all day at high school, and based an assignment on 'love & marriage' on her feelings for me, on webcam she showed me that she'd written my name on her hand with hearts, and said she likes to put her phone on vibrate and leave it in her panties when i text her. Regularly says she wants me in her bed and that she misses me and think about me constantly. A couple of times on webcam recently she seems to be dressing to please too.
I am convinced this is just an intense crush as she's only 15, and I don't think any other guy has payed this kind of attention to her. She is physically mature and very attractive, but as I said I am just uncomfortable with her age. The problem is she seems to have fallen head over heals for me without me really realizing it until now, and I feel as though if I stop talking to her or remind her that i don't want to get involved in that way it will affect her further than just temporary sadness. When she meets another guy closer to her age who treats her like I have could she have problems trusting him? You know what I mean? I don't want to prolong this but at the same time I don't want to crush her. I know it's not really my problem, but her feelings seem so intense and I do not want to be responsible for any emotional or social problems she could have because of me. Is it normal for this sort of thing to happen and for e to feel this way?