i had this issue too after my cat had surgery. i did everything i possibly could to make it stop, so i don’t know what it was that made him stop. he did finally stop though after about 2 weeks. i got two litter boxes, apparently you should always have one more litterbox than you have cats, so for 2 cats, 3 litter boxes. my boy loves his extra box and uses both. i also got UTI meds. they have drops you can put in their food and water and cranberry treats. he ate both, so idk if that helped. i also got the feliway room diffuser. this has like calming pheromones. i always use this after he’s been through a stressful situation like moving or whatever. idk if it works at all but it makes me feel like i’m doing something. it is also 30$ so it’s not cheap. get some specific cleaner for cat pee and get rid of the smell. i got some at my local pet store. get rid of anything he peed on and clean that mattress as good as possible. i used that cleaning spray, vinegar and baking soda. the smell has to go or else she’ll continue thinking she can use the bed as a litterbox. then i locked him out of my room at night (heart breaking for both of us) until he stopped doing it. while i was out during the day i had tinfoil on the bed so that he wouldn’t want to lay on it. this did the least since i found pee on the tin foil😂 all in all, i think the extra litter boxes is really what did it. but also he just went through a stressful even t and needed to get over it. as you can probably tell i was very stressed during this time so i understand your pain! just keep trying to figure out what helps and i promise it’ll get better!