Have you ever wondered why you say the things you do to your children?

I never believed I would hear myself say anything similar to my mother's words until I had a child of my own. The sad thing, you don't mean to say some of the things you say. It's just a heated moment that made you speak before thinking. Then you wish you could take it back. But you can't.
My mind is always working. As a matter of fact you can say that I'm an over-thinker. With that, I start to think about things that I shouldn't. I begin to feel guilty for things out of my control.
To make matters worse, my son asks just a simple question daily, "Is it dad's day?" I have done so many things to help him know which day. Yes, it plays on my mind because I begin to wonder, does he not want to be with me? And to make things worse, he says "Yay, there's still snow. When I go to dad's I can play in the snow!" He is so excited and I break down. Now I feel that I am no fun. I tried to get him to go out with me for two days to build a snowman, but he didn't want to. My mental state today is shaky as I realize that I am" just mom." I will not be the fun one because of the way visitation works. I am the one who has to keep stability. Is that really such a sad thing? To know that when he is an adult the structure I provided for him will only help him build character in a positive manner.
I wish I could keep telling myself to stay strong but I am weakening. I am sure he doesn't mean it how it sounds and if he does, at least I know he is having fun when he is not with me.