Scared I will have no friends at sixthform

Your mentality of thinking you won't have any friends at Sixth Form seems to be coming from this feeling of being left out by your two closest friends. Are these friends you can confide in at all? If you can, then you should tell them how you're feeling and that you feel left in the background. Tell them that you're not envious or jealous, just that you want to be a bigger part of the friendship. As for Sixth Form, you have to be the best you you can be and try to be yourself. I know that's generic advice, but Sixth Form like any other school is full with a diverse range of pupils with diverse characteristics. You should definitely be able to find friends, just be confident. Try to strike up conversations and try not to fear rejection. Fear is what prevents you from moving forward, so do what you can to be the best version of you you can be.

Thank you,
Yes I can confide in my 2 friend but I feel like if I tell them about this their going to think I'm even weirder. As for trying to make new friends its honestly not that easy for me. I'm always the odd one out or the weird one. I don't think I can just go up to someone and strike a conversation yk. I honestly think i'm doomed

They shouldn't think you're weird if you can confide in them. Time will make you more capable and confident, it can take years to gather yourself up and become everything you know you can be, but if you can get over the fear then you'll be unstoppable. You aren't doomed, you just need to keep on persevering and learning. I used to be so insecure and riddled with difficulties and found it hard to make friends, but now I'm a mature adult I've been able to see that I can be the best me, and if anyone has a problem with you-well it's only their problem and it shouldn't have to be your problem.