Jamie, I am the mother of 4 kiddoes and I hear ya.
Do NOT be afraid to claim some "me" time.
It took me a long time to realize that I actually earned respect from my kids when I did stand up for my time. And I think in the long run it made them start thinking of me as a real person and not just their mom. And, also in the long run, it made them more aware of their own time and how to use it to their advantage.
I've now got 4 college graduates instead of 4 small children, and they are beginning to have children of their own. I have the MOST FUN seeing them struggle now with their children.
Although, they do get kind of mad at me when I laugh out loud while they are trying to discipline their offspring! But sometimes I just can't help it!