I've found that I lose lots of motivation when it takes a while to paint basic troops, as it gets quite repetitive quite fast. My advice would be to try and find a model that you really like the look of, and try work on that. It also helps if it is from another army, or just a model that requires a different approach to building or painting. When I started my daemon army, I had heeeeaps of bloodletters to paint, and I think they are now my least favourite model to build and paint, just because of that bad experience, so I decided to take a break from daemons and paint a hive tyrant, just to break up the monotony of my hobby at that time. If painting something new doesn't work, you might just simply need a small break from the hobby, and when you come back you should feel much better about it. Good luck and I hope you find some motivation!
"That which we foolishly call truth is only a small island in a vast sea of the unknown. For man to truly flourish, he must be willing to abandon the ever-shrinking island of such petty 'truth' and surrender himself to the reality of that which is beyond."
Dark Apostle Erebus, architect of the Horus Heresy