Do you plan your practice?

What I used to do (and still get my students to do) is break practice time up as follows (1 hour):

5 minute warmup (spider patterns / sweeps / chords - whatever, just get your hands moving)
10 minutes on scales/arpeggios
10 minutes on chords
10 minutes playing chord charts and coming up with rhythms
10 minutes playing lead over jam tracks
15 minutes playing a song you're working on / your own composition

Obviously you can extend the time depending on how much you have available, and also include more material as necessary - but it's important to keep the discipline on breaking up time. I find the structure helps people get the 'work' done and the 15 minutes at the end is the reward.

If you're having trouble motivating yourself I'd hazard the guess that you're spending too much time on mechanics and not enough on making music... so try learning a song or two, or applying some of the things you're learning to musical situations. Try writing a song... or just jam over some backings - we've got some here if you need them... ('Jam' section in the top menu) :)