I did Carpathian Peak +2 and during the last rest interval I almost cracked. I wanted to be anywhere else but on the bike. When I got to the last work interval I was fine. I always have my weakest moments during the rest intervals, is that normal?
Coming off a hard interval is when my breathing hurts the most because my focus drops a little bit. Make sure that as you are coming off intervals you don’t try to immediately shut it off. Focus on breathing and cadence until you get that first big chunk of recovery in. After everything settles down a little then start eating, drinking, and whatever else you get done in the rest periods.
That was a hard ride for me too. I find the recovery target on most rides too high and often ignore it.
If your cracking and taxing yourself back it off and finish strong
I think that is normal. It is not the rest that cracks you, it is the anticipation of the upcoming interval. Once you get into the next dreaded interval, you get over the anticipatory fear and just keep going knowing that it isn’t really all that bad.
I always try super hard in the intervals that when the rest portion arrives I feel like I am going to fail - and I still manage to bring my game up again for the next interval, mental stubbornness.
I should imagine I am no where near your standard of cycling. I have never used an inside trainer before so it’s good to hear I am not the only one that rides this way 
Workouts like that create mental fatigue the same as they do physical. The fact that you got through it is a great learning experience.
I did the exact same workout yesterday, I desperately wanted to quit during the first interval but after completing that was fine for the rest of them