There's a lot to this post, and there are a lot of options for you. Here's something to chew on...
1) Get Lessons!
You probably feel like you've hit a wall because you have certain basics down, but can't go beyond that, and that makes it tough to learn stuff on the fly. While it may seem like lessons will only rehash the basics, it's quite the opposite. More on that later...
2) PRACTICE PRACTICE PRACTICE
...and practice even more! Practice scales and basic excercises to build up your dexterity and familiarity with the notes you're playing.
3) Practice along with songs
Get something like a Tascam Bass Trainer (I think that's what it's called), KORG Pandora, or any other practice amp that will let you run a line in from your CD/MP3 player or computer. Play along with familiar songs, songs that you've had a hard time with, or/and songs you've been wanting to learn. As you do this, you'll probably find yourself identifying common progressions.
At this point, if you're getting lessons while you're practicing, you'll probably start recognizing not just how nice those fancy runs and riffs sound, but WHY they sound so nice and the theory and pattern behind them. Those mundane lessons can become a real eye opener! As you recognize these things, it becomes easier to practice them, put them together, and you'll start noticing things like "hey, this would sound good right here". That's learning to improvise and the foundation for good groovin'.
4) Practice with your bandmates!
Even if you are capable of laying down the sickest grooves on the planet, if you aren't familiar with your bandmates and their playing styles, then even with your talent it can still result in a hot mess if everone is clashing. Work with them as often as you can, and keep a good eye/ear on them to get a feel for their styles and tendancies.
You mentioned that you've been reduced to playing 2 Sundays, so I'm assuming that you have another bassist filling in on those Sundays. If that's the case, make sure that you're watching and learning from/with that person. During your rehearsals, talk to and work with the other bassist and try to learn as much as possible.
Just a few items to consider. I'm sure others will chime in with more ideas for you. Keep at it!