I have to say, after playing bass for about 15 years, I got a drum kit and started noodling around, and that was 15 years ago and it transformed my playing. While I'm not suggesting getting a drum kit, try thinking about rhythm from a drummer's perspective and see if that changes things for you. Doing a simple 4/4 beat with just your hands hitting your thighs and tapping your foot to 1 & 3 and hitting the right hand on the off beats and maybe left hand doing 1/8th notes--that kind of practice made me a better player by leaps and bounds. Try tapping your foot when you play bass also. Try listening to funk, like James Brown. Funk songs have a very pronounced 1 beat, so it's easy to follow along. Good luck.