how to learn everything necessary for junior/senior year fast?

That's really great but try not to stress about it. It might be worth asking the Head of Year of the school you'll be attending for advice and any study materials. Although schools have started the summer holidays now. If you have seen an Educational Psychologist whilst not attending school they may be able to help you. Or contact the local County Council 's Education Department for advice. I wish you well.

I assume you're in America as you say "homeschooled".
Have you tried to follow any kind of curriculum and where do you think you need to concentrate?

You say you haven’t been able to learn... have the issues gone now?
Could you do part time initially?
Are your parents supportive?

Glad you picked that up Not spaghetti 🤣 I'm not familiar with the education system in America.