quick reaction: the grit you have to complete work stuff maybe has to move to the side project. For example reverse your day: first 2 hours are for your side project. this likely will feel weird and shake up your current strategy, but that's the point!

as we get older we naturally come to focus on less and less whether due to raw physical capacity, growing responsibilities, and even just more sense. So i would suggest reframing the question as how to prioritize and focus my efforts, vs how do i add more capacity for a side project.