The chief prevention against getting old is to remain astonished. 1
This is absolutely something I struggle with. Or perhaps it's more like "maintaining motivation is a skill". Or even identifying what my real motivation was in the first place:
Half of experience is recognizing when you want the thing versus when you want the thing only because you unconsciously want something else that’s a potential side effect you don’t want to cop to.2
Another thing I struggle with. I tend to play out a lot of scenarios in my mind, and I think that tricks my brain into thinking I've achieved what I thought about. Obviously I'm aware I haven't done it, but I lose the urgency to do that thing pretty quickly.
About 99% of the time, the right time is right now. 1
It would seem a good remedy is to immediately translate thought into action. It doesn't necessarily mean I drop everything I'm currently doing, but rather capture the thought in an actionable way (echos of GTD) so I can pick up the thread in the future.