How is your basic aerobic fitness? Sounds like you have been doing some good threshold work. It takes a good level of fitness to be able to hold the intensity and then to repeat it. Greg Lemond said about the pros and I'm paraphrasing... We hurt just as much, we just go faster. Bike racing is a tactical sport, and as much as fitness translates to potential victory, so too does using whatever gas is in the tank wisely.
It's never easy to hold that kind of intensity for too long without crapping out. If you are unable to complete your set maybe drop it a notch intensity wise. Going from 93% to 91% of maxHR can make a big difference and go a long way to completing the workout. IMO anything over 90% is really VO2 territory. Not many folks, even racers, can hold 95% for 5 minutes. In the stratosphere of HR, 1% can make it or break it. You may also want to try your rest in Z2.
Edit: also you don't need to do out of the saddle unless you like it but it takes some good focus to maintain. A lot of training plans put things in numerical terms, and that's essential for tracking load, but in the beginning there's a learning curve understanding how these numbers translate into one's physical discomfort during the effort, and maintenance thereof, and how hard to run the interval at.
