A friend of mine had the same problem when it came time to practice the same skill. What helped her was that she practiced the skill outside of the class environment; we both sat on the steps of the pool (she had a couple of pounds in her weight belt to keep her sitting on the step) with only mask and snorkel, and without attaching the snorkel to her mask. She then removed her mask and continued inhaling through the snorkel and exhaling through her nose until she felt comfortable enough to inhale and exhale through the snorkel. I only monitored the position of the snorkel, the rest she did herself.
Because this was done in the shallow end, it also gave her the reassurance of not sinking while keeping a somewhat vertical attitude, much the same as kneeling on the deep end of the pool does, which is where you will do the skill with the class both in the pool and in open water. The only difference between the class and our practice was the breathing apparatus.
Good luck and by all means, stick with it. This is a fairly minor issue that you'll be able to overcome with practice.
Semper Safe,
Rick
Because this was done in the shallow end, it also gave her the reassurance of not sinking while keeping a somewhat vertical attitude, much the same as kneeling on the deep end of the pool does, which is where you will do the skill with the class both in the pool and in open water. The only difference between the class and our practice was the breathing apparatus.
Good luck and by all means, stick with it. This is a fairly minor issue that you'll be able to overcome with practice.
Semper Safe,
Rick