I've put some thought into my 10 years of musical experience and I've come up with a list of things that one can do to keep themselves motivated to play even dreary, boring things on an instrument.
1. Think about your goals on your instrument.
Not everybody wants to be a pro. In fact, many people take up an instrument purely for points to get into university. Evaluating what you actually want to get out of playing is a big help for time management. For example - is it vital that you practice scales for two hours every day, or can you give yourself a little bit of a breather? Some people say that you can never do too much practice, but I don't think that this is true. I have learnt from experience that over-practicing can lead to this dry spell and also to stress and anxiety relating to performance.
2. Stop practicing in big chunks!
Anything that you learn from practice needs to be retained in your long term memory. The chances are, if you are practicing solely for hours on end, you are not going to remember much from the beginning of your practice, you are going to get stressed and most likely agitated when things don't go your way. It works for some people, but not all. My teacher has always told me that short practices multiple times a day are the best, and I can honestly say that I believe this to be true. Half an hour here and there can soon add up to hours of practice without making you feel tired of playing.
3. Make a rough timetable.
Planning when you will practice what is a great idea to keep you on track. For example; certain scales and on certain days, or certain studies and techniques. For me, I like to practice all my scales and arpeggios earlier in the morning, my studies mid morning, my ukulele and vocals in the afternoon (both classically and jazz), and and other pieces for exams etc. in the evening.
4. Watch an inspirational video of a pro.
Needless to say, this is a very good way of motivating yourself if you wish to be just as good as them. Better, even. Always tell yourself that if you keep practicing you could become the best player in the world, and somebody that your muse looks up to, even. For me, I always find Mayuko Kamio's competition video from Tchaikovsky International makes me want to pick up my violin.
5. Give yourself a break.
If you've been working hard and find it impossible to motivate yourself, maybe you just need a break. Taking a day or two away from practice not only gives you space to breathe, relax and do what you want, but also can highlight the things that you are weaker on when you come back to practice, showing you what to work harder on. However, make sure to keep listening to music, as it might just motivate you to want to play.