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Hi there...sorry to hear that you are having this frustrating problem.
The first thing to figure out is whether this problem is behavioural or if there is a medical component to it. Your veterinarian should be able to collect urine from the cat and will quickly be able to do some tests to tell you if there is an infection or crystals or some type of inflammation that is causing the cat to have urinary discomfort. Cats with any of these problems will often urinate in inappropriate places.
If the urine is normal, then the problem is likely behavioural which can be harder to fix.
This is an excellent article that talks about different causes for house soiling:
One of the common problems that I see is that we are often not good enough at cleaning the litter box. The textbooks tell us that if we have 3 cats we should have 4 litter boxes AND clean them every day. Often, this solves the problem. Another thing I would do is avoid covered litter boxes. Even though they may smell clean to us, to a cat with a good nose they are like a nasty outhouse.
I hope this helps. If your vet determines that this is a medical problem then fixing it is usually easy. If it is behavioural your vet will discuss ways to change the environment to help, and may possibly prescribe some medication to help as well.
Hope this helps!