I've started countless projects all with a wave of optimism on how its going to be different this time, how this will be the side project that earns me enough recurring revenue to leave my job and start making things I want to build full time, but I never can finish them.
I am in awe at people who can work on things for 4+ years without getting large scale feedback or revenue from them. My attention span for a side project lasts a month at most.
What helped me in the past was to write down a single thing that I want to add the next time I feel motivated again and then spend the whole evening focussed on shipping that particular thing. Because I always feel motivated and focussed I can keep my attention span up for several months, previously I would totally get bored after a couple of days.
But reading your comment got me thinking... what about, hiring someone to work on your idea, and then pair-programming with that person?
The (perhaps implicit) idea would be that the person/team would be leading, and you'd be given a clear path to focus on the bits you wanted to and/or had time+energy for.
Squinting at this from the right angles, you could interpret such an idea as a race-to-the-bottom toward project management. But... perhaps that's a direction you could go in?
In any case, what side projects were there?