I want to become closer with my friend but I just don't know how?

Okay so I’m in a group of four friends, and we are quite close. I have been friends with two of the for four years and to be honest I have always seen them as being closer with each other than me, and I was just the tagalong. About two years ago I made friends with another girl, who I’ll call A. A and I met in a class I sat next to her in and we became close-ish and met up a couple of times outside of school, until a year ago we (my two friends, A and I) ended up in the same form. Over the past six months we’ve been getting closer and basically A is now a part of my friendship group, but I kinda want to be, like, ‘best’ friends with her. (It sounds pretty noob-y when I say it like that but whatever) I feel like we’re closer than the others but we still always do everything as a group, and I don’t know if it makes sense but I kinda have confidence issues when it comes to friendship, partially because my whole time in primary school was a social train wreck and the fault was all mine, so when we do things as a group of four I feel like I’m maybe not wanted there and I’m only a part of the group just because nobody can be bothered to tell me to leave, or something. I’m not saying my friends make me feel that way, it’s just that being part of a large group isn’t the same as having a close friend to yourself, because that means somebody actually wants to be your friend and isn’t just talking to you for the sake of it. A was sort of the first friend I have ever had who made me feel like she actually respected and took me seriously, whereas my other friends have always seen me as a clown and someone to joke around with and tease and pretend be mean to. They say ‘oh, you’re just that one friend we can bully haha’ and I laugh along but honestly, it’s tiring and I don’t think I really am the person they see me as, that is, someone who doesn’t have serious feelings. That’s why I really value A, for actually listening to me and complimenting me about stuff and having deeper conversation and just generally acknowledging that, yes I’m kind of ditzy and sometimes clumsy but I’m a real person with feelings just like anyone else, despite the fact that I kind of act like I don’t sometimes because I don’t want people to think I’m annoyed with how they treat me, laughing at me but then expecting me to always be there for them. I love my friends, I really do, but to be honest I’m sort of fed up with them thinking they can treat me like crap and it won’t hurt me because we’re friends, which is why I would like to be even closer with A, someone who treats me better. Unfortunately, being as socially inept as I am, I can’t really work out how to become closer with A without embarrassing myself if it turns out she doesn’t even like me, and if that doesn’t happen I don’t want my other two friends getting upset with me for blowing them off because I really do value their friendship, it’s just that I’ve always felt like a third wheel. I’m really awkward at this stuff but A and I do have a lot of inside jokes just for us, and she comes to me for advice on things I don’t think she tells the others about, but I don’t know if this is just normal for friends because I haven’t really had enough of them to tell, because A has a lot of other friends and two she is especially close with, whereas I just have this four and a couple of others, so maybe she doesn’t want to get any closer, and I don’t want to end up being clingy so I feel like I should just leave it because I don’t really want to lose her as a friend. Also, and this is probably just me rambling now, but I’m bi and to be honest if I start getting closer I’m scared A will think I want to be more than friends even though I REALLY don’t see her that way, I know her too well aha. So yeah, I’m in a bit of a pickle and although it’s probably overthinking a LOT, I would appreciate advice? Thanks xx