This sounds very familiar. I have a collection of T-shirts, too, that I barely wear anymore, and I did throw a load out a few years ago. I'm much more likely to wear something which only portrays my personality indirectly, rather than slogans or references to music or fiction.

I think not feeling the need to shout about one's personality or tastes is sometimes part of "growing up". For me it was also to do with being a bit more interested in clothes and being happy to choose my wardrobe based on colour and style, rather than picking a T-shirt as a default (which is what I did before). I can't imagine that's true for you.

I have designed my own shirts before, for myself and friends. I have an idea right now, but I'm not sure if it will happen.