You cannot overcome inner feelings by outer trappings. It is not that other people are going to walk over you, it is that you are afraid they will; that's very different. So, no amount of muscular body and cold looks can make you feel better inside. You have to get a muscular inside and warm, welcoming looks to get that good feeling.

Start with how well you have been taking care of yourself! You did not cower in the corner and give up. You worked out and tried to protect yourself. The trick is to realize you can do that; has anyone taken advantage of you lately? No. Unfortunately, part of that is because people are afraid to come close to you at all which is probably not what you want?

I would start with practicing smiling instead of looking cold. Hard to do/remember but think about how well you have kept people away. Think of things in their extreme too, so you can laugh. Here you have this muscular body, are a strong, virile 20 year old man and what if a little old lady were to want to take advantage of you? I always think of my stepmother, how she tickled my brother into submission when he was about your age No other way she could have, it was very fun/funny for all of us.

Have you ever thought about how much bigger and stronger you are than your mother, even your father? When my stepson was a young teen and got his growth spurt, he started getting a bit aggressive and we were playing "catch" in a long hallway in my house and I had to gently nudge him against the wall and point out, "You may be taller than I am but I outweigh you by over 100 pounds" :-)

Think of your assets, not your fears. You are intelligent, thoughtful, have worked hard with yourself to help yourself and have, you just did not understand how to do the inside bit. It can be learned and you are intelligent. . . etc.!

There are lots of books and sites to help you learn. I like this one because most of the steps are "easy"/can be practiced right away:

http://www.thechangeblog.com/self-confidence/