How do I make friends as an adult? I can't seem to figure it out.

I've tried joining clubs, attending hangout groups, joining a gym, taking classes, etc. I can't seem to make any friends. I'm not socially awkward, I don't think I'm unattractive, I'm just really lonely. I've had plenty of people say they are interested in being friends with me but they never follow through. I've even had people want to be my friend for a short amount of time but then ditch me for other people. I'm getting really tired of this and just want some actual friends.