Good thread.
I have a lot of challenges in terms of knowledge, technique, execution, all this stuff. I can play decently well, but I can't play the way I'd like to be able to.
The most difficult challenge for me on bass is truly expressing myself on the instrument. I can play songs by ear, I can fall into a groove, I can do a lot of stuff - but at the end of the day, I just don't sound the way I'd truly like to sound, and often, I can't seem to translate what's in my head to the instrument in real time. (Sure, sometimes it works, but I've been put on the spot to solo and I can't express a line I want - or worse, I just don't have any ideas and the solo just sucks. Or sometimes you want to be able to nail that slick two-bar gospel fill - and you do, but you miss the landing! Ouch.) I'm decently proficient, I understand theory, I have a good ear and I'm playing out....but I dunno, I just feel like I'm not all that good.
I think there are things that I can practice that will definitely help me get better - better fretboard knowledge, deepening my understanding of theory (especially jazz-related theory, like chord extensions and "exotic scales"), acquiring new techniques (integrating chords into my regular playing more, using tapping as part of fundamental lines), improving fundamentals (bought Bass Fitness; used to do exercises like these but I stopped a while back, time to start again), and just playing a lot more. Those are challenges in themselves. But I feel as though making the instrument my voice is going to be a lifetime goal, and I don't know if I'll ever be as good as the other bassists that I hear. It's definitely difficult.
That's my struggle.