Why do I feel sad and lonely?

its your mindset , you shouldnt feel so dependent and wrapped in friendships where you don't even realize yourself , detach yourself from friendships and see how much people approach you its okay to be alone , would you rather be in a room of fancy people that you know dislike you or in your own space ?

Yeah I feel that at uni too. I have many acquaintances but don't think any of them would want to be proper friends. I think the gap year and moving on to uni/work would be a good fresh start though, there won't be the pressure of 'everyone's friends with each other already'. Especially with the gap year if you travel and stuff those will be people you probably won't get out of your way to meet again, you might as well try your best to make connections.

Yeah I have heard Uni can be quite hard to make friends and that many people struggle with being alone. I hope your are okay tho

uni will be better, you'll have much more options. There's always one person in a year group who just doesn't really find their people. You've been stuck with the same 100 people for several years. Uni will be much better, and you'll all have done similar a levels, be on the same course, live nearby, etc. Plus they'll be more mature ( hopefully.)