Losing a Loved one is hard but losing a Parent or a Child is even harder but sadly in life as you know everyone passes away and there isn't anything we can do I so wish there were.
We can only do the best we can when we lose a loved one and I believe for myself the only way I will be able to get through it when the time comes is with God and my other family members support and also a few friends. I know without a doubt I will mourn and be depressed but will have to pick myself back up and go on because they would not want us to live in misery and mourn ourselves sick.
You will go on and you will be okay it will take time of course and Praying also learning to do more things out there in life I know you Volunteer that is great but there are so many more things to do for distraction like working a life for you meeting a nice lady or joining some groups like bowling, golfing , fishing, just some hobbies etc.
I know you love her but worrying and planning ahead wont solve anything so live for each day with her and know after death their is life . You will see her again but while you are here make life happen for you and face your own fears because at your age you do still have a lot of years to go . I know you are what 52 or 53? I see you in chat and you are a stinker but a riot and you use laughter to deal with pain I do the same I don't go on and on about my health in chat because it makes me more anxious so I just say I am okay when I am really a full blown mess! I do fear losing my parents and other loved ones I believe we all do.
I miss my grandparents everyday all of them and many other family members but I know I will see them again.
take care and know you will make it through!