You're not a bad mummy, you're a human mummy with a huge load on her plate... and it's not selfish at all, me-time is really important, you NEED it to be able to keep running & giving.
It's like a petrol tank, you've got to put some back in after a while otherwise you run down.. and you know it's not just about you then.

I've been there with my kids... I still get to that point more than I want to admit Trillian is right, better to walk away & let her sook for a bit, and i reckon the guilt you might feel over that is not as bad as you feel now?

BIG hugs