I’d imagine that’s quite a lot of people go wrong - thinking that to get the benefit of sweet spot training it has to be hard. What’s actually happening is that you’re training at or above FTP and getting less benefit due to the building fatigue as you do more and more workouts at that intensity. Eventually there is a plateau and the improvements start to fade.
According to the typical power curves in WKO4, you should be able to hold 90% of 1 hour power for more than 2 hours. Obviously everybody’s different but it gives you some idea of how the workouts should feel.
Mike