Congrats on the 3 weeks sober!
It took me awhile to be happy after I quit drinking. Sometimes I'm still not happy, but life sober and unhappy is a hell of alot better than drunk and wishing to die.
I don't think it's as much as going and doing and finding what makes you happy but more of finding things that you are grateful for and adding to that.
Find one thing today that you are grateful for and build on that. Today I'm happy I'm sober and I can start there and go a longggg way and pretty soon I'm smiling.