Motivation is difficult, especially with a musical instrument. I have struggled with this same predicament. It's easy to find ourselves overworked and not wanting to practice during our free time. I suggest the following: find an inspiring recording, or piece, and set goals. Consistency is the most important with a musical instrument. I encourage my students to keep practice journals. In these I have them write a goal at the top of the page (what they want out of their practice session). This can be a measure of music, a bowing, a fingering, or a concept. I then have them break down what they can do to achieve this goal. For example, work with metronome, play in front of mirror, record myself and the beginning, middle and end of my practice session. I have them spend 10 minutes on each task, filling their practice time. I then have them evaluate themselves, writing 3 things they improved on, and 3 things they need to continue to work on. Hope this helps!