You have some options here:
1. Re-tune your bass from E-C so the bottom string is familiar to you. Be sure to buy different strings so you don't kill your truss rod should you go this way.
2. Play songs you would normally play on a 4 string in different places on the neck. That way, you will force yourself to use the B string for bass instead of as a glorified thumb rest.
3. Learn new songs that require a 5 string.
+1 on option 3. This will be the biggest help if you're learning to play on a 5.