If I’m having a bad practice session, I stop whatever I’m working and pull out my scales. Doing scales forces me to slow down and helps with my intonation and finger patterns. It’s like a reset switch. After 5 minutes or so of scales, I return to the piece that was causing me such grief. Usually, I’m able to get through it much easier…simply because my fingers and muscle memory have been tuned up. Other times, if I’m really frustrated, I do like Michael mentioned. I look back 6-8 months to what I was working on back then. I realize then that I’ve come a long way in every facet. Playing older, easier pieces also helps me realize how far I’ve come. Bad days are a given. Use them to your advantage by reflecting on the progress you’ve made and it will turn into a good day.