WOW!!! I stumbled on this thread and took a while to read through it. A lot of good points were made and some not-so-good. Though I'm sure Estrella has already been on her vacation, I'd like to throw in my two cents for others who may have her condition and want to take up diving.
First, once I figured out that you were a vet with PTSD I knew a three day course in Hawaii was not for you. When you asked about muscle memory I knew what your mind-set was. I did fifteen years in the Army and was in Desert Shield/Desert Storm. Though I was never diagnosed with PTSD, I was "edgy" for a couple years. After a while it subsided and I have no problems.
What you may want to do is find an experienced instructor who is a vet i.e. someone who you can relate to and can relate to you. You don't need SEAL type training, just someone who understands and will take the time to teach you. You definitely need private instruction or a small class and you should disclose your condition to the instructor before you start as well as a release from a doctor.
I know there is a medical patient/practitioner confidentiality rule. Contrary to iDoc's comments I think your instructor should be forewarned so he can anticipate any problems or, decide not to teach you at all if he's not comfortable with it. Don't worry, someone out there will teach you.
Ultimately, you should be comfortable with the instructor. The instructor will make you comfortable with the skills so you will be comfortable in the water. As an instructor, I have taught people with physical disabilities and non-swimmers. Since I teach in Korea, I'm not on a schedule like tourist areas such as Hawaii or Thailand are and can afford to take the time these students require. I'm sure in your area, you can find an instructor who will take the time you need.
I think Jim_Lap's comments are worth reading again. Personally, I see no reason why you shouldn't learn to dive if you can get the release. I'm not a doctor, but I think diving may help you to deal with your PTSD. Just my opinion.
One more thing, from a Vet to a Vet, thank you for your sacrifice in your service to our nation.