If you write on the guitar, try tuning the strings into an unfamiliar tuning. You will start hearing sounds you like rather than sounds that you might think are ‘right’ or ‘wrong’. I find doing this helpful, particularly when I start to get frustrated about doing the same thing over and over on the guitar, or too caught up in music theory. It gets me back into my creative right brain. If you play the piano, try picking up another instrument!