I had a similar problem. It was shortly after exploratory surgery on my throat to find the primary so my throat was still swollen and I had a hard time exhaling. About 1/2 way through the fitting I couldn't exhale at all and panicked. We worked with repositioning my head to get a clear airway and I managed to dig down and get through it. They also cut the chin away on the mask so it didn't feel like pressure there. Between those adjustments and valium I got through my 35 25 minute treatments with a few anxious moments but, as has been said, soothing music and counting the positions also helped. Work with the techs to find what's comfortable for you. The alternative is not, imo, acceptable.