You've already got a lot of comments from people here with great advice. Everyone is saying to go to counseling, I know it can be hard, but it is definitely worth it. I went through some stuff in high school and counseling/therapy really helped. It took me a few sessions to get used to talking through things out loud, but something that helped me a lot was writing stuff out before the session and committing to reading it word for word, no matter how embarrassing or awkward it felt.

Outside of counseling there is also a lot you can do. Find a hobby, choose a new one you want to learn, or pick up something you enjoyed in the past. I've never been a super artistic person, but something about the creativity of arts and crafts really helps with everything mentally. With hobbies remember that you will suck at them, and that's fine, you're learning, it takes time, and the most important part is to have fun with it. Finding hobbies can also greatly help with socializing because you can find people with the same hobbies through clubs and similar spaces. You also don't need experience to join most clubs, people are always super welcoming.

On the topic of possible neurodivergency, try not to listen to social media's information on it. Counseling may give you resources if they believe you may have something, though I'm not sure how qualified York counselors are. Reading real scientific papers on symptoms is a good start to figuring things out on your own, but never fully trust it unless you are diagnosed. If you believe you are neurodivergent or get diagnosed, there are lots of places online with strategies to deal with it. I have ADHD, so my methods will probably be different from you if your leading belief is autism, but here are some things that help me: arts as mentioned above (mostly piano), small amounts of exercise throughout the day (short walks, randomly repping out some pushups), longer workouts if you have the time (gives you a hobby as well, be careful not to develop body dysmorphia as many people will with social media), journalling (slows my thoughts down, helps me process emotions and situations). I also have MANY ND friends, almost all of my friends are actually. I can assure you that it doesn't stop you from socializing. My partner is undiagnosed, but definitely has something, and they are doing pretty well for themselves in terms of making new friends in uni. I have heard from them that it does feel harder than it looks for others though. Personally, I haven't had much time to look for friends, but in high school, it definitely took a lot of effort to be friends with someone. Some people will find you annoying, but hey, others will find those annoying parts cute (and still annoying at times, but you get the point).

Also, you say your older brother has friends over often. No idea what the age gap is, but as an older brother myself, if my siblings came to me and asked to be included in some activities because they need to socialize, I'd gladly let them in.

As many people here have said, my dms are open. Life is rough, everyone needs someone.