Yes, I have had days like that. I hate feling that way, but it happens.
Think about the impact that you have on your son, your family, friends, etc. Surely what you have done and accomplished doesn't mean "nothing". Think about those times when you smile at a complete stranger when you are walking down the street, you may have made his or her day and not even realize it. Yes, we are insignificant in some ways, but in other ways we impact people's lives in ways that we are not even aware.
Anyway, that's what I tell myself when I feel blue.
{{{hugs}}}