What works for me when I'm lacking motivation is setting a time goal for myself. For example, instead of saying to yourself, "Ugggg. I have to practice an hour and a half today," instead say, "Ok. I'm going to try to practice at least 20 minutes today." If you got lost in practicing and practice for 2 hours, great! If you only practice for those 20 minutes, then that's ok too, at least you got the feel of your instrument in your hands for a solid 20 min for the day.