When you feel like shit,train lighter,but with less rest..............this way you are still very intense.
I`m talking 30-45 second rest periods between exercises,depending on the movement and the bodypart.
Do this and you`ll be hurting so much,you`ll have no choice but to stay focused/motivated.
Use a weight that makes you work for 8-12 reps or more,again depending on the bodypart being trained.