If I get a music idea (like a little melody or something) that I really like, but I forget how it goes by the time I get home... will it eventually just pop back in my head, or is it gone forever?
you need to write down new ideas immediately. new ideas are the most forgettable, because you've never heard them before. it will vanish as soon as you hear another song. there is no guarantee that it will come back to you, and if it does, you will never be certain if it's the same. you need to catch them immediately, if you take yourself seriously