It’s difficult to assess the seriousness of this on the basis of only a letter so let me just share a couple of thoughts.
People by nature tend to fall somewhere on a scale from introvert to extrovert. It may be that at least part of this is that you are on the introvert end of the scale. There is nothing wrong with that! But, the larger culture celebrates extroverts. In high school, the general idea is that everyone is supposed to be outgoing and popular. Of course, not everyone can be. Not everyone wants to be. So let’s at least assume that part of what goes on is that you have a more restrained approach to life. One thing that supports this is that you do have a couple of close friends and that seems to be enough for you.
As for being out with others: I wonder if you have developed the skills you need to be comfortable out in the social world. Introverts can and do learn how to manage social situations enough so that they can be successful in their careers and can find loving relationships. Notice that I said “skills”. Skills are behaviors you can learn (like how to make small talk, how to be an active listener, how to be a good communicator, etc.). As you get more practiced in these skills, you will be more comfortable being with people for longer periods of time. But you will still need your alone time to “recover” from a lot of social interaction.
The line in your letter that does worry me is your statement that you can spend hours holed up in your room and in bed. Yes, you may need time to recharge your social batteries but it would do a lot more for your self-esteem if you would use that time to pursue an interest or to do something to make the world a better place. Think about ways to support a cause or candidate or to educate yourself about something you are genuinely interested in. You will feel better about yourself than if you just hang out wondering if there is something wrong with you. An important bonus is that it will give you more to talk about when you are with people.
I hope this is helpful. I wish you well.