Have you thought about taking a class? You could take a photography class or cooking class or whatever you're interested in. That way (A) you're being productive even if you don't meet people you want to be friends with and (B) you're more likely to meet people who share common interests. The reason why it was a lot easier making friends as a kid was probably because you saw the same people over and over. You got to know them before you became friends. You most likely didn't go up to random kids and suddenly became friends with them.