I'm in love with my best friend and I don't know how to handle it. Does anyone have any advice?

We've been friends for a year, and we've mostly been talking online, even if we live in the same city. We speak a lot every day, we're very close I would say, but I'm not sure whether she considers me her best friend as well. She's my favourite person. She's also bisexual, like me, but she's in a serious, monogamous relationship with a man. She would like to be polyamorous but her partner doesn't want to. I'm not sure if she would be interested in me anyway - I think not.

I'm crumbling after every time we meet IRL, I start missing and wanting her very badly. I've cried a lot over her. I get upset that she's kind of cold or absent sometimes, when I need her to offer support, and I'm not sure whether this is because I'm in love with her, or because I need more than that in a friendship anyway. I also feel that I "see" her less the more I want to be with her - I feel like I can't recall many things she told me about herself because I am so preoccupied with being obsessed with her, which I really dislike in me but that's that. 

I don't even know whether to tell her that I love her or not. I'm afraid I would feel resentful if she refuses me, which I know would be shitty of me, but I can't help how I feel unfortunately - and it would certainly result in me avoiding her and possibly fracturing the friendship, which I value a lot. Or that she wouldn't want to be friends anymore.

For context, I'm genderqueer, she's a cis woman, I consider it a lesbian crush. I'm autistic/adhd and possibly BPD too. I find it very hard to stop obsessing over people and am very emotionally unstable.