i have ADD and take medication. i have found that taking medication does not affect my creativity at all, if anything, it helps it. it helpes me focus more and remember things. i have also found that taking medication has greately improved the quality of my life.
if you want to get better at writing songs, you could look at the lyrics of bands you like. i have written many songs, and find it a good outlet to vent my feelings and the things going on in my life.
good luck with your band!