Why does my female cat keep peeing everywhere!?

I'm starting to go crazy! My female cat has been peeing everywhere for the past year. I have taken her to the vet at least 5 times in the past year for this. At one vets place, he checked her urine two different times, and both times said she had a UTI. I gave her the meds he gave to me, and she didn't get any better! The two other times i took her to the same vet, he didn't even check her! He gave her the same diagnosis and same meds, both times she didn't get better. I decided that maybe this vet didn't know what he was doing, and took her to a different place. They couldn't check her urine there because her bladder was empty, they checked many different things and said she was healthy, just a little too plump and needed to go on a diet. He also said that he thinks she has a urinary disease and gave her special canned kitty food to help with it. It did not help her! It also seems like she is in heat constantly, she meows all hours of the night, and I cannot sleep. She pees on anything!!! If my pillow falls onto the floor in the middle of the night, she pees on it. My parents wont allow her upstairs at all because of this problem. I have her in my room at all times, her cat box is available to her 24/7 (i have my own bathroom in my room and the door stays open so she has her cat box, and her food and water available at all times) She never used to be this way. Shes my baby, i don't want to give her away, but after last-night i cant help but to think that's what i need to do. I'm lost. :( I love her so much, and I have tried everything except getting her fixed. My room constantly stinks of cat urine, i feel like I'm cleaning all the time just to get rid of her messes. HELP!!! Would getting her fixed keep her from peeing everywhere? It's an expensive surgery, and I can't afford it, but if that's what needs to be done, then I'll just have to borrow money from someone.