Hi Renee - When I first started nebulizing with Tobramycin (for Pseudomonas - they had to get that "out of the way" to treat my MAC) that was how I felt.
The crusty old pulmonologist didn't seem to care, but his very helpful nurse set up a nebulizer with just a little saline and monitored my technique. She told me I was working too hard!
She taught me to sit comfortably in a firm chair - no nebbing while slouched on the comfy sofa. This fully expands your lungs and your chest cavity. Put a glass of water nearby, relax with a few deep breathes, bring the nebulizer to my lips and breathe NORMALLY. If it started to hurt, she had me pause, take a sip of water and a few more deep breaths, and restart.
She also told me - no clenching the mouthpiece in your teeth because it tenses up all of your muscles when you do that and you are setting yourself up for problems. And no trying to take extra-deep breaths to "get the mist deeper" - gravity will do that for you. It took a little awareness and practice, but I haven't had the problem again.

When our "littles" first needed to neb (as 2-3 year olds) my daughter to me to stand them between my legs so their lungs were fully expanded to do their treatments, this made sure their lungs were fully expanded.

How do you sit when you neb?