I really like this girl but I can't be with her? I don't know what to do?

There's this girl I've known for 3 years and I've always liked her but not as a crush, I just always thought that she was really pretty. This year we started talking and when i first spoke to her i was shy but she smiled and gushed because i had spoke to her. We had classes together and we'd help each other with our work and she'd always sit close and if our legs or arms were touching she wouldn't move. She brushed me hair for me one day and whenever we talk about our problems she confronts me and is really sweet. She said she's had a dream about me and she likes and comments on my pictures on instagram. She has a boyfriend though and she talks to me about him sometimes. I texted her one day and she had thought i had forgot about her and she was thinking about me. Earlier in the year we hungout and the next day she said she had a dream about me. Anyways she said that she wants to become closer and be able to talk about anything. I'm unsure of my feelings for her, she's very beautiful and sweet and at school she always smiles and speaks to me and hugs me if i ask for one. She used to take pictures of us together too. My friends want me to tell her about how i feel but i dont wanna ruin our friendship, i know she used to like girls but i can tell she really loves her boyfriend.
I really like this girl but I can't be with her? I don't know what to do?
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