My husband went through the same thing about 6 years ago. You will regain your confidence and begin living fully again. Yes, it is a blow but these human bodies are truly remarkable. There is certainly grief as we get reminded of our mortality. Rejoice in your body and it's ability to take care of itself and you brain's ability to cope and adapt. This too will pass

As an 80 y.o., I try to foremost Live in a way that I'll not be looking back at with: But did you not do what was in YOUR control?
I believe while a lot is outside our control -- we came in this world which was here before us, with its constraints -- so We need to live within those as you rightly said of our genes and environments.

But there is still a lot up to US to Make our life how we wish it. In my case, it's how I eat, my physical activities, how I deal with stresses whether emotional, physical, or even existential/spiritual,

In my case while I've 'lost' in some ways, I also Gain in other ways such as with more nuanced understanding of what This life Is. In other words there is no point in comparing what I used to be to what I am, Even if there were serious missteps. I cannot afford - at This age! if I must say so though it'd be true for any age -- that our time, each day and hour is a gift that would be gone forever. So I TRY to live the life I still have in ways that are best for me, with open mind that allows for growing: learning by using our inborn curiosity, social bonds and playfulness. That may mean changing my ways to what makes a Little Better sense.
I wish you are living the most of Your Life.