Why is OCD so common amongst humans and even animals?

It’s said that up to 1 in 50 people have it, but I would say it is much higher, maybe 1 in 20. Why have humans evolved to have so many mental difficulties like this? Even animals seem to suffer from it (excessive licking in cats, grooming etc.) My sister, me and my mother all suffer from it amongst many other problems. I get angry because I don’t understand why.

What is the most simplest organism or form of life that displays these symptoms? And why is the need for perfection or “equalness” the centre of this disorder? How does evolution account for this? I feel like there’s so much wrong with me physically and mentally that I’m not meant to be alive, like I’m an anomaly in nature, not fit for life. Sorry I’m getting off topic. Wow, I’m so fucked up :( Counselling, advice and medication do nothing btw.