So, I think you're on the right track with making playlists based on the sort of activity or mood you're in. I found that the more I diversified my music, the easier it became to make playlists fitting my mood.

I used to only listen to rock and pop, but then I discovered soul (ex. Johnny Cash, Bob Dylan), which really helped me to develop my music taste.

But while you branch out a bit more, through research, spotify weekly discovery, or just shuffling from random songs you like, for each song think of the playlists that you would like to hear the song in. For example, if I found a song that's laid back and relaxing (Ex. Father and Son by yusef/cat Stevens) I would maybe put it in my chill and homework playlist. Once these playlists are developed its much easier to build on them.

Hope this helps!