Training and testing is more difficult indoors than outdoors for a number of reasons. Most people’s FTPs are significantly lower indoors, which suggests that separate testing in both environments is required to ‘set’ them for training purposes.
Failing that rigmarole, you can experiment indoors by setting it say 20 watts lower than your outdoor FTP and see how you cope. If you still can’t get through your Sufferfest intervals, drop it down another 10 watts etc. until you can. You will be able to raise it later as you get fitter and better at your indoor workouts.
Make sure you are doing your workout fresh and pumped. Provide yourself with the most motivational music available and ensure that you’ve got plenty of fan action happening to keep you as cool as possible and to reproduce the stimulus of air rushing past.