Trainer/Sufferfest question

I've been on the trainer twice now using sufferfest. As a relatively new cyclist, my riding abilities are really weak. When it comes to sufferfest, I have only been able to complete about 2/3 of it before I hit the wall and drop down to almost no resistance and spin for me warm down. My question is , should I reduce the intensity to be able to complete the workout or should I maintain the intensity and my endurance will follow?

If I were running, I know I would reduce the intensity to try to complete the duration of the exercise, but I often see here that when riding you should ride often, mostly hard, sometimes easy which makes me think I should maintain the intensity and the endurance will follow.