Motivating yourself to do anything can be really difficult sometimes, I know the feeling of not wanting to put the work into something, or wanting to achieve something but it seeming overwhelming, and sometimes it can feel like you just want to give up. 

I've been there and done all that, and it's not a nice feeling.  So I've come up with a few suggestions to motivate yourself when you're perhaps going through a tough time.

Listen to as much music as possible - find an artist or a style that you really like, find something that inspires you.

Listening to music is key to understanding how to get the best out of your potential musical ability.  Think outside the box, find out about styles or artists you never knew existed, you'll never know what you discover. 

I'm a big fan of video games, and recently I have discovered a wealth of video game music that I absolutely love, and this is what has inspired me to pursue a long term career as a video game composer. 

Also, go to concerts, there is nothing like watching a live performance of anything, you can't beat it.  It doesn't necessarily have to be concerts based on whatever you're studying either.  Try going to see a theatre play, or an opera, or a rock band.

Take a break from the instrument

If practising for long periods of time just isn't working, try doing things in stages.  For example, you could do your warm-up then have a 5-10 minute break, then play through one of your pieces, take another break, then work on a particular section.  I remember from the Guildhall open day I attended, Eric Crees said to me, "it's always better to do 15 minutes of focused practice than an hour of half hearted practice.

One thing that I like to do while practising at the Guildhall is switching between the piano and the trombone, or while I'm at home the guitar and the trombone.  Other things you could are listen to a recording of a piece you're playing, or just listen to any music that you really like.  You could also do something completely unrelated to music, I know someone who excercises in between practice sessions or before concerts, he says it clears his mind.  Find whatever works for you.