Why?

Why can I feel ok one minute and then one word and BOOM my world crashes again?  Why can I turn so suddenly from ok to sad and crying with just a question from my son?  and that question was do we have ice cream?
My mind then suddenly screams "NO WE HAVE NO ICE CREAM BECUASE YOUR MOM IS A LOOSER AND YOU CAN'T HAVE NICE THINGS."
My mind then starts the wirlwind of you are a loser, see you can't give the kids what they want.  you are a dissappointment to all everyone.  Nobody wants you why do you bother to even make yourself better?  no matter what you do you are crap a nobody.
Now comes the fight to get back to ok again.  
How does this happen? I was fine and then the wall drops and everything comes crashing down again and now I have to try and build the wall again so I can at least get through the day tomorrow.    Not seeing my therapist until Thursday but why can it be like a switch?  I was fine and now battling the negative demons again.
Or is it just in my head?  am I just being selfish thinking poor little me there are people worse off than me and I am complaining whining perhaps this really is as good as it is going to get for me.
I will become old and still be single and lonely with no one about to hug, nobody to lean on no one.