I agree with the other posts, you have been grieving the past 13 years already, grieving that he had his sickness, and grieving knowing he's going to die some day. How much you grieve is not a measuring stick on how much you loved him. We all know you LOVED him, and I feel you know you loved him too. Keep writing, it all makes perfect sense. Love you!!
...again, you help say all the things I feel but cannot say. I know my grief and guilt are different from yours and yet how you voice it feels so familiar. Thank you . All you feel is right as you have said. There is no right or wrong.
You loved him, but also grieved the loss of the boy he could've been without that devastating disease. I imagine it seems easier because you've already grieved in your own way.
The way I think about part what you are feeling, although I have not had your experience, is that you have already grieved his loss. You spent 13 years loving him knowing that one day he would be gone. You gave him a beautiful life. You did everything you could. After so many years it is just so hard to get used to what you have been preparing for for so long. No one is ever ready no matter the circumstance. Each situation is unique so we can't compare ourselves to others and how they grieve. It is ok not to be ok. It is ok to feel what you are feeling. It doesn't mean you are not missing him. You are his mom and know that he is free. Please keep posting as I know it is very healing and we are all here for you.