Donald, for a 58 year old you are running quicker than anyone else I know. ..and most people way younger than that. Trish says you are amazing.
Maybe you are just getting older like the rest of us. Unfortunately.
On the other hand maybe it's too many Ashvale whales. :-)
I agree with Roger - you are training and racing (cross country, roads etc) like a 28 year old not a 58 year old.
I find interval sessions now take me 3 days to recover from (and I'm running a lot slower than I was). I suspect you are trying to do too many quality sessions and the delayed recovery is affecting your major race performances because you are not fully recovered.
Try training with maybe 1 top quality session a week and a second shorter or slightly easier session - e.g. one park run or full interval session plus a shorter hill session or an interval session at a lower intensity. See if this has any appreciable effect.
Its good to reflect on what might be affecting your performances and your list covers most of the options I consider for myself.
I'd also suggest you try reflecting on the fact you are still able to get out in such fabulous places as this last weekend and be proud of that fact almost regardless of the performance itself. Try and enjoy that for what it is a little more. Finishing at the very sharp end is certainly not the only thing. I'd have given up long ago if it was :-)
Thanks for the comments - wise words for me to take heed of over the next 4 months.