Inability to Urinate!!! HELP!?

Hey, I'm a 17-year-old female, and I have been having a HUGE problem that has persisted for over a year now-- chronic urinary retention (or repetitive inability to pee). It started when I was 16. I had driven to a soccer tournament and REALLY had to pee in the car... except when I got to the tournament, I wasn't able to go. Nothing I could do could help me pee, I couldn't even get a couple drops out. I played the next soccer game and tried not to think about it, but the pain in my bladder became so extreme that my dad rushed me to the hospital. I was catheterized, and tested for UTI, but all the tests came back negative. They simply gave me antibiotics, and sent me on my way. I thought the problem was fixed-- I was able to pee just fine for about a week-- but then one day, when I REALLY had to go again, I absolutely couldn't. I sat on the toilet for hours, and was finally able to get it out, bit by bit. This same scenario has repeated tons of times-- I have to pee, I can't, and I have to sit on the toilet until I can go-- and now these episodes are becoming more and more frequent. :(. The strange thing was, is that I had started taking Yaz birth control about the time that these episodes began, so thinking that that was the problem, I went off the pill... But nothing changed. To help myself pee, I've tried ice, warm baths, running faucet, etc., and nothing really seems to work. I just have to wait excruciating hours until I can pee again. But recently, two months ago, I was on vacation and was faced with another retention episode. I thought this would be just like the others, and diminish after a few hours, but it didn't. It became so painful, that I had to go to the infirmary and get catheterized AGAIN. By this point, my parents and I were freaking out. About a month ago, we saw a urologist. He gave me a prescription for Vesicare-- a drug that lessens the feel of needing to pee, but that hasn't helped at all. I still pee about every half hour of the day, if not more, and the episodes haven't quit. He gave me a cystocopy (a procedure that analyzes the anatomical make-up of your bladder, to check for physical problems), but THAT turned up negative, too. Suffice to say, this defect is RUINING MY LIFE. I'm afraid to leave home and go on car trips of vacations, because I'm afraid that I'll be faced with another retention episode and won't be able to go. It's painful, and I really don't know what to do. Is anyone dealing with a similar problem, or is there a professional, that can give me some legitimate advice?? I'm running out of options, and since I've had this problem for over a year now (and with it getting progressively worse) I feel that my bladder may be facing permanent damage. PLEASE HELP!!