You heat up a LOT indoors on the bike. Without a good fan, durations and intensities can suffer.
Invest in either a decent box fan OR if you can afford a decent "blower". A blower is the thing companies use to dry out a room after flood damage. Looks like a big plastic turbocharger with an oval shaped outlet.
Other possibility is that you're overestimating your "ftp" or whatever value you're basing your workouts off of.
If you chose wisely, a workout should be a challenge but should still be able to be completed. That sounds very vague, but to an extent "you just know when you know".
If you're getting out of the zone in the last 5min of a 2x20 set......just finish and grit your teeth. If your finish point goal is like 165 bpm but you're at 175 bpm by the 10th minute, you calculated too high of a % to try.
Finishing all sets of a workout is more productive than bonking 1/3 of a workout all the time. If a workout seemed easy, add 5w next time. Or 10w.
I will say, a lot of the sufferfest and other stuff seems like a weird peppering of zones and intensities and durations and not very repeatable workouts all based around an ftp or "base value".
If I have some 2 sets of 3x3 and I can't do any of the 2nd set or can only get like 2 1/2 minutes...I can decide myself to back the power off next workout. Or, if I bonk the very last rep in the last 30 seconds........close enough.
Something about good ole intervals that's appealing. You can track progress and make modifications easier.