Your tilt table test was positive for POTS [postural orthostatic tachycardia syndrome] from what you describe. If fludrocortisone alone is not working, you may benefit from taking it along with a vasoconstrictor such as Midodrine. As AireScottie mentioned, some people self-medicate with nicotine because it also constricts blood vessels; the principle is pretty much the same with prescription medications taken for their vasoconstrictive properties. The more you can get your blood vessels to constrict, the more blood you can ultimately keep flowing to the right places (your heart and brain), and the less sick you will feel.
Unless you have high blood pressure (in which case, disregard this or at least talk to your cardiologist about it before making any dietary changes), in addition to increasing your fluid intake, you want to increase your salt intake. Dysautonomia experts are recommending about 5000 mg a day of sodium for POTS patients. If you haven't already been checked for electrolyte imbalances, you may want to ask your doctor to give you a blood test to check for those, particularly hypokalemia (low potassium). You can increase your dietary potassium intake by eating more fruit and vegetables such as skin-on potatoes, lima/pinto/kidney beans, lentils, and spinach.
The biggest triggers for POTS are heat, exercise, and of course, standing up. For this reason, AireScottie is right on when she discusses the history of the disease in the military. During the civil war, it was called "soldier's heart."
I highly recommend you read some of Dr. Blair Grubb's introductory materials regarding POTS and syncope which should give you a better understanding of what you are experiencing. You can find links to them in our health pages; here's a direct link to the health page where they are.
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
Whether or not you will be able to continue your job will hinge largely upon how severe your symptoms are and how well you respond to treatment. I know of two women who were unable to sustain their jobs in the military due to POTS, but both of them had regular bouts of syncope that did not resolve with treatment, so that may not be relevant to your situation.
Hoping the best for you. Keep us updated if you can, and let us know if you have more questions.