It's a question of finding a way to eat the elephant one bite at a time.
You've got a complicated skill that's hard to learn. You have to figure out how to break it down into simpler skills that are easier to learn, and then build back up to the more complicated skill.
Ideas, most already mentioned above:
- practice the bass line separately.
- practice the vocal line separately.
- learn just one bar at a time.
- chart out how the bass line and vocal line rhythms interact.
- slow it down as much as you need to make it easy, even if it seems ridiculously slow.
- practice tapping the two rhythms on a table with your fingers, instead of singing one and playing the other.
- tap the bass rhythm on a table while you sing the vocal part.
- forget about getting the right notes in bass line, just rest your fretting hand across all four strings to mute, and worry about getting the right rhythm with your plucking hand.
You've got to break it down into the tiniest pieces you can, until you find something that's easy to learn, then learn that. Be slow and methodical, and you'll get back up to the more complicated thing sooner than you expect.
And with practice you'll eventually find you need to break things down a little less, and can go for more complicated things from the start.
But I always try to err on the side of being slow and methodical at first. You'll get there.