Why can’t you get your old brain back once OCD has been triggered?

Like if you went from normal thinking to OCD brain, why can’t your brain just go back to normal over time?

Does it just forget everything it’s learned up to that point, and all your previous regular thinking patterns?

Especially those who develop it later in life, does the brain just forget 30+ years of functioning without intrusive thoughts? I don’t get it, but I miss my old brain so much :(