I don't know all of the answers to your questions, but can give you some sense of assistance by telling you how therapy has helped me.

--Is there any way to actually fix this or is it something I am stuck with?

Personality Disorders are considered life-long. That doesn't mean that you can't make changes in order to be happier and less avoidant. It's very hard work w/a T who is willing (and knowledgeable) about what you need.

--can someone have this even if at earlier points in life they had more friends and did more things with people??

Yes. I was a social butterfly in HS and dated in college. I had very good friends in my past, but eventually problems in my childhood (of which I was mostly ignorant about) caught up and surfaced as serious cognitive deficits. What you might think was no big deal is a big deal...

--How is this different from being an introvert?

I'm an introvert - very sensitive. I don't know the answer to this question, except to tell you that Personality Disorders present w/signs that are different than just being introverted. This would be a good question for the T you choose to go through therapy with...if you so choose that route.

I wish you the best. If you go to therapy, even w/the an exceptional T, you'll have a rough go of it - fear of abandonment, contact outside of therapy issues, lots of surprises about yourself that you don't now know. It's been worth it for me in the past 2-1/2 years. And many times I've wanted to (or almost) terminated. The best advice anyone on PC can give, I think, is to find the right T. You'll hopefully feel it when you do.