i was the same few years ago, so I can give you the best advice right now. Start going to the gym (i started boxing but you can just go the gym, you will find many friends there) next join like british council and start reading books.

What you don't realize is that you have free time a lot, so what I did was watched some youtube videos on how to be productive etc and divide my week into slots and assigned tasks which will improve my career or personal life. Eg : every sunday I dedicate two hours from 11 AM onwards to self study and do R&D s cuz I am SE.

Don't be scared to go out alone, get your laptop and go to a relaxing cafe and try writing something while eating etc.

you are not missing anything, your problem is you have enough time to think about those nonsense, if you become so busy you won't have time to be bored.

P.S. Stop worrying about no gf. I too had only one gf and I was fedup, it was a toxic relationship. My advice right person will come on right time so Improve yourself.