It depends on the illness. If it's OCD it's going to feel completely difference than gender dysphoria, social anxiety or some other issue. I can only speak about OCD. Imagine doing the most tedious thing you know time and time again until it feels right. Placing a glass down in exactly the same place, clicking buttons on the computer with the cursor in an identical position, standing in each section of the living room an even number of times, blinking twice every time you see a word you like and all sort of stupid things. You know it's irrational and doesn't make a difference. Yet you do it anyway in case it does. Imagine thinking that and every time you notice you haven't done something you need to go back and do it in exactly the same way otherwise death results. I was turning on and off my laptop because the point on my arrow button wasn't exactly at the top on the vertical line in the shutdown symbol and I couldn't align it within the 4 second time limit I set myself. Now imagine doing that and it had to be perfect on an even number of attempts. I spent over an hour trying to nail it. I felt like breaking something at the end of it. The frustration kept building. I couldn't pull myself away from doing it. I thought I would have died if I didn't.
All long behind me now. That should give you an idea