... or the EKG says I have a heart arrhythmia, then barring some serious mix-up, I can be certain my arm is broken or I have a heart arrhythmia. Mental health diagnoses can be a little looser than that.
If the new information in my possession is useful to me, I use it, if it's not I file it away for the moment and try not to disturb it. Sometimes it's not easy. I've got an OCD diagnosis too, but with me it's always been pretty obvious. I've had it so bad that it does interfere with my life. I'm prone to wake up at night grinding along the same circular paths in my mind that lead nowhere. I've been trapped by my OCD in places where I stop eating or sleeping because those activities would interfere with whatever I was obsessing about.
The useless obsessions don't go entirely away with the right meds, but they don't interfere so much with my life. Most nights I can sleep well enough. But sometimes I worry I'm missing out on the very rare sorts of obsessions (I can count the times on my fingers) that were useful, that allowed me to solve the problem that no one else could solve. But I've also solved these same sorts of problems without having them stuck in my head, in gentle restful sleep, wake up refreshed and excited by a new approach to a problem. So maybe my perception that the OCD was ever beneficial in this way is faulty. But I also know the OCD keeps me going when everything else has gone dark. If I don't exist then I can't do the useless things I have to do. Therefore I continue to exist no matter how dark my world has become.
I got a surprising PTSD diagnosis and the therapist suggested EMDR. I've always felt pretty solid here, I had a very secure early childhood, a foundation strong enough that not even the severe bullying I experienced in middle and high school can't haunt me, it's simply part of who I am, the story of me. The worst traumas, the PTSD stuff, came after I quit high school.
I've never done anything about it, no EMDR or other sorts of therapy. My "triggers," the sorts of situations that set off a panic attack, are rare and usually easy for me to avoid.
Good luck to you!