For me at least, with 3 kids at 7yo, 4yo, 1yo... when I say "I'm tired" as an excuse for not getting to what I want it truly means both that i'm physically tired, and mentally drained. Between work and kids right now, I might get an hour or so per day of mental/physical downtime. Which is okay, because the stuff I'd like to do is like... reading tabletop rpg books and playing videogames and such. Which is all stuff I can do when they're older, too. The stuff that really matters is the stuff they're doing, playing and learning and all that. So I give what I have for energy to the three of them, since I can always catch my goals up later.
You only have yourself to blame! ;D
There are some things that can be postponed, some things that can't. How I see it when it comes to children, we have to also ensure that we take care of ourselves and demonstrate how to be a healthy adult that has things to do, other than playing on the floor. I have a feeling that many of us are spending too much time with our kids, which may hinder them exploring life for themselves.