What does everyone do if they lose motivation to speedrun?

Not with that attitude. ; )

I run. Forget sex, running is the best thing ever. You don't ever disappoint someone and you can do it whenever and for as long as you want.

but it do

Just take a break. Play through a game casually if you're sick of speedrunning as a whole. Learn a new run if you're sick of that game specifically. It's just a hobby for most of us, so there's no real reason to push yourself to the point of not enjoying it anymore.