I understand that money may be tight but it really is unfair to leave your cat in pain and obvious distress. She actually has rights under the animal welfare act, and by leaving her in this pain you're technically breaking the law. Firstly, get her to the vets ASAP, the sooner she gets treatment the better. Then get her insured, it won't cover you for this outbreak of cystitis but it will cover her for other illness which is inevitable as she ages. Finally, put aside some money each month for emergencies.
Not trying to scare you but one of my cats had a blockage in his urethra and it cost over £1000 to save his life. My little 1 y/o kitten has crystals in his bladder and is on a special diet to control his problem and avoid him having major surgery. Pets are expensive, that's something you have to accept as part of their ownership.