I am new diver but have had similar experiences as you describe on at least two occasions. In both instances I was using rented gear that did not fit well. One thing to check is the fastening straps and cumerbund on your BC. If these are too tight they can prevent your rib cage from expanding when you try to fill your lungs with air. The sensation can cause a great deal of anxiety which can then cause you to gasp for air making conditions worse. A "too tight" weight belt can have a negative impact as well and you may want to consider a BC with an integrated weight system. I have also had this occur when wearing a wet suit that was too small, again rented gear. Finally, diving in cold water, sounds like you do with a 7 mm suit, can cause a similar event. I dive in the Bay of Maine and the temps approach freezing. When first in I wait to overcome the "panic sensation" of freezing to death while the water in my suit warms up before descending. In closing the best advice I could pass on is to purchase your own gear from a qualified shop if you haven't done so all ready. While I still "choke" in the freezing Maine waters on entry the other issues have been taken care of by getting gear that works for me.