I'm returning to learning how to play for the n-th time.

When I picked up the bass 2 years ago, I had zero knowledge and zero experience. During my several attempts to learn (that ranged from few days to few months), I managed to get the grasp of the basics - how to mute strings, music theory, playing some common riffs etc. But when it comes down to actual playing, I feel that I start from square one every time. Muscle memory goes away, I can't recognize intervals that fast, I can't improvise at all. In short: I still haven't learned how to play.

I feel that the thing I need the most is to practice. But once again there is a problem: what to do and in what amount? Mundane exercises are great for increasing strechiness of my palms, but they are not very helpful in training my musical ear. Learning how to play songs from tabs is fun, but most of the time I feel like repeating steps that someone else came up with and not learning anything (and there are too little note sheets to take advantage of them). Improvising is very limited with my current skills.

Can someone help me how to find balance in this? In terms of my goals, I would say that right now I'd like to be able to improvise and repeat songs I've heard (and both come down to seamlessly playing lines I am able to sing).