What is your experience with nebulizers?

Can anyone explain how you are using your nebulizer? I have been prescribed levalbuterol administered through my nebulizer. I no longer have my Pulmonary Dr. and plan to discuss this with my new Dr. on Thursday. How often do you use yours? I understand it's used to loosen phlegm but when I do use it? I cough so hard and the phlegm is very difficult to cough up and I feel like I'm drowning. I quit using it until I am clear as to when to use it. My last Dr. said "as needed". When do I "need" to use it? I am so confused.