Could it be that part of the reason you're not doing the thing is because you tried to do too much all at once? 

I did this too when it came to Whole30 and running marathons and the 100 day running streak! I took on too much at once, couldn't keep it up, and went way back to 0.

Wanna maintain motivation? Wanna be the person who actually can keep doing the thing? Just do something a lil outside of your comfort. Know the steps you have to do. Get your reps in, and then do something a lil outside of your comfort, repeat, repeat, repeat.