Hi S.. I am so sorry for you. I know how damaging these can be as I witnessed them with my ex husband for years. His started in college and we miss diagnosed until he was in the Marine Corps 7 years later. He went years in between attacks and then would have several a day for 1-2 weeks. They came on just as you describe. He would be walking along and out of nowhere, he drop to the ground and try to back up into anything solid. The fear on his face was horrible. He would hyperventilate and sometimes pass out. I kept paper bags with me at all times. I told people that it looked like his worst childhood fear had come to life and was standing over him.
They prescribed therapy and meds, which seemed to help. It took months to get him over these attacks as he became incredibly agoraphobic during and immediately after. I took the caring approach during and the tough but gentle love after to get him back out into the world. The good news is that they seem to go away, each episode getting father apart as he aged. From every 18 months to 3 years to 5 years and now, as far as I know, he hasn't had and attack since a few months after we divorced. You 'age out' of them, so to speak.
One trick we used was keeping a rubber band on his wrist (like overcoming an addiction or bad habit) and he could fiddle with it, or snap it to remind himself that he was ok, that it wasn't real and that he had control during an attack. Another technique was to ask him how bad it was (scale of 100 so that I could talk him 'down' in small increments). If he said 50, I'd say Ok, feel yourself coming down to 49...to 48...etc. If I was going to fast or too slow, he could tell me. At a major number, like halfway through I'd tell him to rest and notice his surroundings, that he was in control, that he could make the anxiety and fear go away. That seemed to really help, especially when he learned to do it on his own. He pictured it like floating, and as he got control, he 'felt' the ground under his feet, until he got you one, when he was stable and 'standing on the ground, in control'.
It is just terrible because you can't handle the activity around you when it is happening, so one person coming to help seems like 10. They thought he was having some kind of heart problem and all of the staff all over him really freaked him out. It was good once we figured it out. I also made him business cards explaining that he had Panic Disorder, what was happening, that no ambulance was required and how to help (keeping activity and sound low, counting with him, etc.) so that he could give them to someone if he had them in public.
Please see someone. Don't be afraid of meds, they can help. But if nothing else, find someone in your area who has dealings with Panic Disorder. There are also books and web pages. If nothing else, educate yourself and let your family know how to help you. Please don't drink...it doesn't really help, just masks the symptoms and can make it worse. Find the path that is right for you and take charge. it is good to know what you are dealing with. It is very scary when they think you are on drugs (as they did when my ex was in college) or Crazy (as they did for the first week in the Corp). It is a very disheartening thing to deal with, but you CAN control it and it will hopefully go away in your late 30's.
Good luck S....I hope this helps.
D.