I think level 45-50 was always a killer for me too. Until the prospect of Outland comes into view it can be hard to keep yourself going. Best thing I've found is to start a new team for a few levels and take advantage of rested XP on the main team now and then to keep them levelled. Alternatively you could just take a few days off to avoid total burn out.

Once you get to about 51-52 you'll be focussed on hitting Outland so that will get your motivation going again.