You can actually cut a little slit in the mouth tape, so that you won't panic. Breathing a straw sized amount of air through the mouth if you need to, will prevent suffocation anxiety, but also limit your mouth breathing enough that your nose does most of the work.

Watch this guys two videos on a DIY CPAP. https://www.youtube.com/watch?v=g8JzXEoT9LI

I can see how it can cause some anxiety. I suppose I was lucky that it didn't for me. The first two nights I work up in the middle a couple times, decently alert, and could tell my body was trying to say something might be wrong. I was able to fall asleep quickly though, which also was something I wasn't able to do before.

If you might have that anxiety, I'd say try putting tape on your mouth during the day at times to get used to it, and you might find you'll forget about it and that can ease the worry. It might be difficult, but we all can breathe only through our noses, even if that requires some practice and reassurance.