My best friend thinks she has a chance with me but I don't want to hurt her feelings?

To make a long story as short as possible, basically my female best friend (19 years old) thinks she has a chance to be in a relationship with me (20 years old), but unfortunately I am just not attracted to her in that way. A few years ago, I was in a relationship with a different girl, and we broke up (it was a mutual decision, though I was pretty sad about it at the time). After that my girl best friend admitted to liking me. Since I was sad at the time, I did tell her that I might have had feelings for her too, but I was coming out of a breakup - my feelings were all over the place so it wasn't a good time. I told her not to wait/expect anything from me as I needed to get over my ex before I could really consider another relationship, so I was okay if she wanted to look around for other guys.
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.
My best friend thinks she has a chance with me but I don't want to hurt her feelings?
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