In the beginning, here is what worked for me and I have used it on others also. It may work for you also. Its pretty simple but it worked quite well.
First off, get permission to use a local swimming pool. Preferably one with a "deep" end.
Take your gear to the pool, dress out, and go to the bottom of the deep end. Now that you are there, spend a LOT of time (or as much as needed) to get use to being underwater. Sit down, lay down, roll over, stand up, play around. You may need to use two tanks. You may need to come back a second or third time. The point is you need to get use to using the gear and being underwater.
Another "tactic" that I have used for years and still do to this day is upon reaching the bottom, I will lay on my stomach for a few minutes and adjust my gear, check my gages, etc. I noticed many years ago, while river diving, that laying flat on your stomach, on the bottom, calms the nerves tremendously. Its amazing how well this works. Try it and I am sure you will agree.
A mental thought process that may help is this: You have a straight shot to the surface. Given even the most extreme emergency there is nothing in the world that can keep you from going straight to the surface and surviving.