That's a great book indeed and has helped many, myself included. I suffer from a great fear of rejection. It doesn't have the hold on me like it used to, but it's still around. I've spent a number of years dealing with emotional struggles of mine and others I care for. If it's any use, I made the most progress after I realized, with the help of others, what I consider to be the absolute primary source of my challenge:

I am addicted to the way I feel.

By that I mean I am addicted to my personality. I could be feeling happy, sad, angry, depressed, "normal" (whatever that means), etc. It doesn't matter. I'm addicted to it. Give me more of it. As much as I can get.

Once I realized how true that was, and that I couldn't change being addicted to my personality, I dedicated myself to changing what I was addicted to. Not perfect, but over time, it worked. And the good news is, being reluctant to be around a lot of new people, I did most of it on my own.

Anyway, I can see in your post that you have some feelings that you certainly don't care for. But I also see there's a lot to like in your posts, especially the part about going out of your comfort zone. Good for you. Hope this helps.