I think it's harder to make friends as an adult. People have families, significant others, children, jobs. I'm really sorry you have been initiating with your coworkers and they have not seemed to be interested. That is definitely frustrating, disappointing, and a crummy feeling.

I have been in the same boat before, and here are things I do: take it one step at a time (sometimes there are lulls and other times there's a flood of things / people happening), do things on your own (like go to meet ups, a club you're interested in, or take a class in something you've always wanted to learn...branch out from your coworkers). You said you're an introvert, but I don't see these things as too hard to do as an introvert. I am an introvert, and sometimes with new groups and clubs, I hang back, due to my own energy level. No one minds. Other times I'm right in there. It just depends on the crowd.

It sounds like from your post you might be doing some of these things. I have 2 words of advice. 1.) Don't try so hard, like don't focus on it so much, but still go out, and 2.) Maybe examine yourself and see if there's something you might in fact, unintentionally be doing that may not be helping you during your social interactions. There is a book called "Stop Being Lonely." I forget who it's by. A woman. Perhaps it might help.

Anyway, good luck. I hope my post helped at least a little.