It is the way of grief Heidi. It is that slow process of coming to terms with the unthinkable. Each does it in their own time and in their own way but at some point we all have to find a way to carry that grief so we can move forward and not be held down by it.
While Mom is still here you need to stay in the moment with her. Yes, she is going to do things that will make you laugh and that is how it should be. I truly enjoy the time with my Mom and we both laugh and smile and have fun with each other. If you look back at what was or forward to what will be you are robbing yourself of those precious moments in the here and now. My Mom is in the moment. She doesn't remember her previous life and had no idea what is ahead. All she knows is what is happening in the here and now. I take my cue from her. Perhaps it is the one thing our loved ones have to offer us.... learning to appreciate what is right in front of us without the past or the future interfering
Just take one day at a time and try to grab the best that day has to offer. If you need to cry then do so. After that good cry, get up, brush yourself off, and look for the silver lining that is surely there.