Tommo, are you trying to come up with your bass lines on your own, or do you get together with the guitarist when putting a song together?
I find that working with the guitarist helps coming up with ideas for bass lines. He'll have something recorded already, he then goes through the song, showing chords and changes, I'll work through it and come up with something, then it's recorded on top of the guitar.
Maybe it's the band situation that's also getting you down, especially the drummer & singer situation. Maybe you should get together with the guitarist & have a discussion about things, see what he thinks of the situation, and whether he wants to look for new members, or carry on with the drummer & singer. Maybe after the discussion, you can make a decision whether to carry on with the band. If it's not working for you, it's not working.