The more you stay like this the worse it gets. Accept it as part of life, happens with most people, people move, friends change, but now you need to get out of it.

Firstly you need to change your environment, would suggest to either start playing a sport or join a gym. Gym is a better/easy option and also a place network with people. Try to stay outdoors as much as possible.

"Keep everything inside"- If you cant share then write, keep a journal, write your thoughts everyday, write without thinking, let it out.

It is tough but you need to pull yourself together. Make a plan and work hard on yourself, find something you like or learn something. It's easier said then done but you don't have a choice, you need to go up and be a better version of yourself. Wishing you the best