Perhaps your mother, being a generation older, has dealt with old age decline and death before. Also, your mother is likely thinking about her own age decline coming up. This can be very sobering. I'm pretty sure if I was 20 years younger when my mother faded out and died, then I would not have been able to handle it as steadily as I was able to 2 years ago. Perhaps your mother is not 'cool' but rather 'steady' in this situation. Remember she has had very different life experiences than you, and more of them. You will be in her position one day, and you'll likely be more able to get through it with more strength then than you are able to now.

It's always so hard to see a loved one slipping away, I'm sorry! Crying is helpful to get us through.