It's already been said, but let me emphasize that the secret is seeing the song as a whole. I had played piano and sang for MANY years before I picked up my first bass. On the piano I often have to play a melody, a counter melody and a bass line while singing something between the melody and counter melody. The "secret" is that it's all part of one song. The concept of only having to play a bass line while singing seems ridiculously easy and it is if you are playing with a band or even to a track. If you try to practice just the bass and vocal alone I would expect that to be difficult because you don't hear the whole song.
Gerry
Gerry