For most of my life, I have found change unsettling and sometimes very worrying, so I do know how you feel.
Fortunately, most of those "sharp corners" 40th birthday, followed predictably 10 years later by 50th, retirement, all turned out to be less worrying than I thought.
I imagine we all worry about the propect of needing more help as we age, or not being able to manage on our own.
I have found been practical (provisional power of attorney, pre-paid funeral plan, will in order) helps,
For me it is harder to accept that to a certain extent it is necessary to slow down, but I have found it worth while to make myself accept that I can now longer mow all the grass in the garden in the course of one afternoon, or clean the whole house thoroughly in the course of two days.
In order to get this far, I have had to sit down an work out exactly what my concerns and unconcious worries were, and how to tackle them.
Life is still worth living, even if the pace is necessarily slower and some of our loved ones are no longer with us. Try to remember that we tend to hear of all the lonely old people left to fairly incompetent carers, BUT we don't hear of all those who have friends, good carers, and few or no health problems.
Why worry about whether you or I will be unfortunate later on? It might never happen. and if it does ´, remember the old Glasgow saying, "We'll be all right, hen, we're Clyde built."