For me, it's most difficult when I'm feeling drained via depression.
If there's some sort of emotion, be it anger or anxiety (and of course brief times of happiness), I can write, it just fuels writing.
When I'm simply feeling low, or in that place of no emotion at all, when I attempt to write and come to a stumbling block, I just start a new word file, and write any old crap. If it feels hard for you to write, writing random stuff will just flex the writing muscles in your brain.
Like, when it's hard to feel or do anything, any movement is a push.