Sorry to hear about your struggles. Knowing early in the day that you’re not going to enjoy yourself is a real buzz killer, happened to me in a 50 mile ultra, I know after 15 miles that I wasn’t going to be finishing let alone walking it in…
I used MV full distance plan base-build-speciality and found it overall manageable. I replaced some of the longer Z2 rides with outdoor rides as we had a very mild winter and spring. Some of the two hour midweek rides I swapped out for 90min workouts (or did morning/evening double) as to be honest two hours was just too much to fit in. About four weeks before CeltMan this year I was finding it challenging to be motivated to do the longer rides, but I think you just get there eventually with long course training.
My bike pedigree doesn’t sound like it’s anywhere close to yours. I wonder if there is another external factor(s) affecting your training and performance? How has your resting heartrate, weight, general mood, libido, diet etc been? Do you have kids? Are you still truly motivated by an IM PR or has it become a stale goal? Is it part of the process of trying to qualify for Kona? Lots of variables at play here!