What's wrong with you? Not a darned thing, you are just doing too much. Stuff you don't have to be doing yourself.
Why can't the children pick up their own toys.... they brought them out. You are their mother, not their maid, they can pick up after themselves.
Entertaining the kids? The older one can entertain the younger one for a while to give you a break.
What's the worst thing that will happen if the tv stand isn't put together? The world will not fall off it's axis.. it will continue to spin... The fence doesn't get weatherproofed today? It can wait a few days. If it's just painting stuff on a fence, have the 8 year old do it, maybe she'd want to earn a few bucks... or turn her into a little Tom Sawyer...
You don't have to do everything yourself, and it it doesn't get done, it doesn't get done...
Don't fret about it.