When I read a letter like yours, I get very concerned that our culture is making everything into an illness. Having opposing feelings at your age is very normal. There is a natural push in all of us to get out there and grow (the extrovert half). There is an equally strong pull (by the introvert side) to want to hide under the covers and not have to deal with all the great many big decisions you will have to make in the next few years: What to do after high school; Who to be with; What kind of job you want to have; How to eventually set out on your own; What kind of person you want to be. These are big, big questions.
The fact that you are stressing tells me that you are a sensitive person who is paying attention. It makes sense to me that you are stuck and feel like you don’t want to do anything. Some part of you thinks that it’s how you can prevent yourself from making a bad decision. Unfortunately, it really doesn’t work that way. No decision ends up being a kind of decision — usually a bad one.
My suggestion: Make a list of the decisions you have to make and start doing what you need to do to check them off. If college is in your future, start talking about schools with your guidance counselor and parents. (Don’t worry about declaring a major right away. Most schools expect that the first two years are a time when kids figure out what they want to do for work.) If you need to get a job out of high school, start volunteering, get an internship, or start a part time job to get some experience.
Enjoy the times that are extroverted and enjoyable. Take a rest from the world when your “introvert side” tells you you need some time to be alone and to think.
I wish you well.