As weird as it sounds, my motivation sinks inversely proportional to the number of tabs I have persistently open in my browser. I think it's due to paralysis of choice when it comes to reading up on a certain topic, leading to being overwhelmed with the perceived about of stuff I need to learn for this project or that.
Solution: Close all tabs and start from a blank slate.
Not sure if that's applicable to your situation, but I hope it helps in some way.