Getting slower with age is expected, and even to be embraced as yet another challenge. But this... basically I feel like I've gotten mentally soft, and that is creating an existential crisis.
Not sure if any cheer leading will do much good. Anyone that's ever gone the distance - whether its 26.2 miles or 29,000 feet - ultimately asks themselves why? And whatever answer they discover in themselves has little to do with what others think.
Surely you have enough experience to know why age and circumstance brings limitations. But you also have the wisdom to know that it is when you give your brevets your fullest measure - your accomplishment has the most meaning and satisfaction.