Not at all EvilCorgi! This is rants and advice and it's designed to rant about things that upset you, so you're not wasting anybodies time *huggles*
So lets start with the sexuality, I have two tips for you.
1. Sexuality is NOT a discrete thing, it's usually very fluid, you may find yourself partly attracted to females and mostly attracted to males, or partly attracted to males and mostly attracted to females. It doesn't make you straight, or gay, or bi, or anything, because it's very "gray" area. In general, you may be attracted to a whole lot of things and trying to classify yourself can be very difficult, so try to keep in mind that sexuality is fluid and not necessarily discrete.
2. Don't worry too much about where you "fit in" in the whole sexuality concept. This is derived from the first but in an essense don't try to classify yourself too much, just realize that you're attracted to whatever you're attracted to and there's not really too much beyond that. If you can classify yourself and you want to, then do, but don't worry if you can't, we're all unique, just know that what you're attracted to is what you're attracted to.
It's a shame none of your friends are willing to appropriately discuss that topic with you, but there's a whole bunch of online resources you can use, like the fandom for example. Discussing it with people can help quite a bit but at the end of the day it's all about finding out who you are and what you do, we can help but it's up to you to actually find it.
As for the education, you don't know what you want to do yet, that's ok. Some people (a lot of people) don't know what they want to do until the YEAR they get into university, a lot of people don't even know after then, not knowing what you want to do is normal, it's just a matter of trying new things and seeing what subjects you enjoy. Maybe it's computer science? Maybe Physics? Maybe Economics? Maybe carpeting? It doesn't matter, it could be anything, it's just a good idea to pursue your interests and finding those interests by trying a whole bunch of new things.
You don't feel like you're doing well enough. There could be a lot of reasons why, for one, some subjects like mathematics require a lot of work and repetition so making sure you put in the work is a good step, but you've said that you've tried and it hasn't helped to a great degree. The other method is to ask for help from a teacher or possibly get a tutor! Trust me, actually pursuing help is something a lot of people aren't brave enough to do, but if you do it you'll find yourself getting much better grades and understanding the content a lot more. The other idea is to collaborate with friends, stop working alone and study with your best mates and figuring out how to solve problems together, when your friend doesn't understand something that you do, explain it to him, when you don't understand and he does he explains it to you. Two minds is better than one, that couldn't be more apparent when studying in a group.
I hope that helped somewhat!