It happens to me while I'm in the process of making a song. It sucks so much cause it can make you fear continuing on with the song. It get you to the point where you just going back and forth with sounds and stuff " Oh no this sound should be change " or " This isnt turning out right " or " I should probably change that " etc. I've found it helpful to either take a small break, eat something, or in general do something that makes you happy or relaxed, and the n come back to the song. Usually it helps a lot to really understand and know with a full heart what it is you want your stuff to sound like. The same problem happen to me, but I've found that since I've realized what I really want, it tend to happen less often than what it used to be back then. ^^ SO dont give up, just keep a clear, serene heart and mind.