Where to start with learning bass guitar?

Hi everyone! I've had my bass for a few years and on-and-off tinkered about with it, learning a few songs, but in true adhd perfectionist fashion I've never got further than this because I didn't feel like I was getting better no matter how often I played. This time I want to give it a real go, and I was wondering where people would recommend I start? As I said, I'm at a stage where I can play a few easy songs (think Ramones, Peaches- The Stranglers, Stacy's Mom, etc), and I know some basic music theory from school and playing guitar, but none specific to bass playing. I can carry on picking songs I like and learning them from tabs, but it just feels like I'm not really improving that much as a player, or as much as I think I could be if I found another method to use alongside this. 

Any tips/advice would be really appreciated, thank you!!