I myself like to be inspired when I'm mapping and at times that can be a challenge, especially if I'm not that "into" what I'm working on. So, my advice is to work on something small that you can finish quickly and you would be interested in. Perhaps a deserted island for instance, or a one man space ship. Usually, if I start something small I end up going way beyond after I get rolling. That first line or stroke seems to be the key.

It could also be you need to do something other than mapping. Try going for a hike, or doing something physical. Perhaps you already do that but if not sometimes just do something opposite of sitting and drawing you will find your energy levels get reset.