I ended up realising I didn't like her as anything more than a friend, though I appreciate what we have as friends and didn't want to ruin it. At the time I didn't tell her that I didn't have those kind of feelings for her because I felt bad (though looking back I wonder if it would have been better to tell her). We got into a small argument months later and she basically said the reason she was upset/annoyed with me was because she still has feelings and I did told her I didn't feel the same way and apologised.
Nowadays, I feel like she tries to act like my girlfriend (texts me gn/gm every time; gets upset if I forget, always puts herself next to me when we are out in a group etc, gets a little clingy when other girls come up).
Despite all this fake girlfriend stuff, she has been an amazing friend to me. I know if I get into a serious relationship with a girl, the relationship with my best friend will have to change (no constant texting at night, can't be clingy with me) and I'm perfectly fine with that - I'd be more comfortable right tbh. I want to still be there for her, like she always has for me, but I know she'll get very upset.
Thanks in advances for the responses.