Hi i'm a very reluctant expert about cystitis, unfortunately.
My record is 14 bouts in a year!
The over the counter sachets dissolved in water help make it much less painful to wee, so i would get some of those.
However, they do not cure or treat the cystitis at all, they rely on masking the symptoms while your body sorts itself out.
If you have had it for over a week and it is not going you do probably do need a short course of ab's to shift it, so i'd go to your gp. They can do an instant urine dip test to check it is a uti and then give you the tablets.
Cranberry juice/tables are now not recommended for uti infections, as they may actually make things worse so avoid these, even if people keep telling you to take them!
When you say it is getting worse, how so?
The danger with a uti is that it travels upwards and causes an infection in the kidney- this is called Pyelonephritis and it is very serious. The symptoms to watch out for are back pain, shivering, temperature and being / feeling sick. If you feel like you have these go to A & E immediately, and take an overnight bag as if it is Pyelonephritis they will keep you in for at least a couple of days to give you IV antibiotics.
If you haven't ever had it, it is hard to understand just how miserable a UTI can make you, so sorry for you rubbish week (Sorry if TMI but it is also a lot less painful to get into a warm bath and pee in there, also allows you to get more pee out, iyswim)