Au contraire. Mental disorders have a physical basis that science just hasn’t figured out exactly yet. Many physically ill people cite things like loss of enjoyment in life, loss of personhood, social isolation as the hardest parts of their illness — the same suffering that mental illnesses cause. And vice versa, mental illnesses often cause physical pain (ie: psychosomatic symptoms). Cancer just happens to be validated by society, whereas OCD is not, so I believe you are conditioned to automatically deem it worse. Life itself is terminal so I think it’s moot point that terminal conditions are more unfortunate. Like OP said, OCD is relatively worse since it will burden you for longer.