Baby steps. Start on dry land, move to the water.
Get acquainted with each piece of gear, that will reduce the amount of discomfort piling on top of your phobias. Fins, BC, regulator, gauges, etc. (dry land then water).
I find that when I concentrate on what I am scared of, my fears distracts me from being able to do what I really intend on doing (example, when you’re up high somewhere you always hear people say "don’t look down"). Our instincts are usually pretty accurate, so if I entertain myself with something else (don’t look down, just look straight ahead) the rest comes unconsciously and instinctually. Before you know it, you’ll forget your fear and you’ll realize "whoa, I’m doing it". The psychological part of the task isn’t as tough anymore when you shift the focus in another direction. Maybe there can be an objective to remove your mask, unzip your BC pouch if you have a pouch on it and take out something from the pouch, maybe a small toy or puzzle that you can blindly put together.
Hope that helps. =)