It's tough. My dad passed away 15 years ago, but he went fast with a heart attack doing what he liked to do until the very end. I can't imagine what it's like with a long drawn out illness. The grieving seemed to come and go in waves until it finally subsided. You never stop missing them and in the end you remember the best of times, not the bad ones.
I just appreciate my mom is still with us at 96 doing the best she can. And all I can do is whatever I can to help her along.
Sorry for your loss.