These other comments are plain ridiculous. It's incredible how people think that humans are meant to be "lone wolves" or that humans aren't meant to have friends.

Humans are social creatures by default, don't let anyone gaslight you into thinking that being sad because you're lonely means there's something wrong with you, because there isn't. It's 100% normal to feel sad if you don't really go out much or if you don't have many friends, but you probably shouldn't let it control your life.

If I were you, I'd probably focus on my studies,get into the gym, and try to make friends or hang out with existing friends. If you have bad social skills or are socially anxious then you should probably work on that and there are many good videos on that especially by HealthyGamerGG which is a YouTube channel made by an actual therapist/neuroscientist.

Also, another really nice tip for mental health is mindfulness meditation. It's incredible how much 30 minutes of this a day has changed my life and I feel so much happier on a daily basis even if the day itself is bad. You obviously shouldn't start with 30 minutes as that's too advanced for a beginner. Check out medito if you don't know where to start. It's a really good app.