What if I don’t talk about my mental health at work?

I’ve been diagnosed with OCD and a physical illness that makes me forgetful. It’s also known to cause brain fog and emotional instability. I’ve been told that I should disclose my condition(s) to my employer in order to protect myself from discrimination. I’m currently applying for jobs but I feel absolutely horrified at the idea of disclosing my mental health issues at work. How will it affect me if I keep it secret?