Can washing my anus during istinja or shower break my fast?

I have a really important question. I tend to use excessive water during istinjā and shower due to an extreme waswasah problem. My question is that (because I have read this) water entering into the front and rear private parts via washing during istinjā or shower really breaks the fast. I feel I can somehow wash the front private part three or four times in a way and still somehow avoid water from going inside my front part, but I am still really unsure about the rear part. You have written in one of your answers that if water enters as long as the suppository reaches, the fast would be invalidated. Can you please tell me how long is a suppository? Is there any excuse for someone like me? During shower, I have great doubt that water enter the rear part because I wash it thoroughly and might slightly allow the water to enter in an effort to make sure that it is properly washed. I draw apart the cheeks to allow water to reach the anus for proper washing. Please guide me as to what should I do.