I've also been finding I can't ride as hard as I used to so it has affected that too. It is my hope that this can be managed without medications or procedures as my doctor told me that I should be able to continue running if I watch the HR as I have been so I take it I may not be in too bad of shape.
You're in the "wishful tinkling" stage of aging. The bottom line - your heart has become older and is signaling its stress by failing to keep rhythm during efforts.
My heart lost rhythm several years after having a valve replaced. I "babied" my heart for several years and avoided any additional treatment other than beta blockers.
Inevitably - I slipped into full blown constant AFIB last spring, I had a Maze procedure done and I am back in normal rhythm - but this isn't a cure - its just a chance for a failing heart not to beat itself up as quickly.
Like yourself - I wanted to ride long and hard - but eventually reality sets in and wishful thinking subsides.