If you lose motivation doing X, it may be too hard or too easy.
In these times, best to use the 10% rule. You'll be in 'flow' if the thing is 10% harder than your skill-set.
This 10% rule is only a heuristic; its purpose is to help narrow down the goal, make it more specific, more actionable.
It doesn't matter whether it's actually 10% harder or 20%. You should be motivated enough to just do the thing.