I'm not sure I can add much to what has been said, but if you haven't you might try adding some distractions, like have a radio going in the bathroom, turn the sink faucet on, anything that might create a kind of distraction but also switch up your rules, negative feelings, and associations around the issue.
What has your therapist suggested? If anything, did it help? Did you mention this to your therapist when it began or just recently? It's a pretty serious symptom that you need good help resolving as quickly as possible, even if it is a little overwhelming to change this pattern. As mentioned above this sounds like it tips into control-like behaviors somewhat similar to OCD. Or I think about all the fears and rules I had around my eating disorder. It's all a way of having control over the body but also over emotions.
With the eating disorder, trauma triggers, or panic episodes, it has helped to push myself outside my comfort zone but only small amounts at first, and with what felt like tolerable steps. So if you can use some of the suggestions above, you could switch up some of your associations, but also challenge yourself to take care of yourself. Adding just a little more fluid and maybe allowing one more trip to the bathroom but finding an alternative to wiping, like mentioned above. Since it's been so long for you with this pattern, there is likely the trauma memory, but also compounded by your own pattern of tight control, which possibly adds to the fear of deviating from what you've been doing.
I think of working with a nutritionist while recovering from anorexia, and doing immediate changes, though with as little overwhelm as possible...but I had to make those changes, even if scary or uncomfortable. Then we added a little more the next week. Distractions and making certain steps easier helps...and you have a whole lot of suggestions at this point for making this all a little easier. Which ones can you try? So, I'm partly curious as I read all of this, and your fear-based resistance to suggestions, how your therapist has helped you address this, maybe from a behavioral perspective? (like eating and getting through that discomfort, or any other necessary survival behavior that challenges our control over our body)