I had a very personal and difficult experience with this particular mental illness and it was very severe type of OCD. The one I was dealing with is called Pure O OCD which means pure obsessions or obssesive thoughts only. The beginning of the disease is very subtle, you don't notice it until it had already taken over your life. For me it all began when I started doing philosophy and personal development. I loved philosophy so much in that period, my whole life was revolving about doing philosophy. I used to enjoy it so much and get so much insights that was changing my whole life to the better, well until the OCD has taken over. My own mind started acting strange and very stupid. It started gaslighting me everytime I adopt a new belief. It started bullying me basically but you trust it so much because it has the same voice as your own mind. You see the OCD has some kind of authority over you, because it is inside your own mind. The OCD is very anal and stupid. If you say this is red, it will say "are you fking mad, this is green". Another very good example is a thought I had once. I started thinking why we say how are you to others when we are not really concerned about them. I knew this idea was stupid but it was persistent and it bothered me for at least a day. The answer of course is that how are u is a kind of greeting. Anyways, this is just one idea, the OCD has thousands of them. It has no specific topic to debate you about. It literally can bother you about anything. I really struggled for years just to understand the things I said here. Your brain becomes so weird and dysfunction that you cannot even know how it was before it became like this. The treatment of OCD ia mainly medical at first then you learn to basically ignore it and to rob it from its mental authority over you.