I have been diagnosed with borderline personality, OCD that is mainly obsessive, social anxiety disorder, and PDD-NOS, which falls into the autism spectrum. My disorders are very intertwined.
My OCD primarily presents as racing, obsessive thoughts. You remember that thing that you did that was really stupid in second grade that was really stupid and your entire class laughed at you? You remember how you screwed up real bad with that one college application four years ago? I remember those things constantly and incessantly. It's torture, and at one point I developed a coping mechanism in which I would hit myself in the head multiple times to try to stop thinking.
Social anxiety makes every little thing equivalent to jumping off a cliff. I constantly obsess over whether I'm conducting myself properly according to society. Ordering at a restaurant? The waiter will laugh at my order. The waiter will hate me because I forgot to smile. The people next to me will think I'm too fat to eat a burger. The cook won't cook the burger through and I'll die of food poisoning. I'll forget my wallet. I'll get sick from the burger and throw up in the middle of the restaurant. These boots will make me trip in the middle of the restaurant and everyone will laugh at me.
I'm high functioning in terms of borderline personality, but it basically means that I require constant proof of people's affection and approval. I constantly analyze and overthink other's actions, trying to find a reason where I screwed up and how the hell I'm going to fix it. I keep "proof" (usually trinkets, letters, messages, etcetera) of good things that happen and good stuff that is said to me in order to prove to myself that these things happen. I cut for a very long time (though not anymore), which is very typical of borderline, and I used to shop compulsively because the feeling of happiness I got was similar to receiving "proof" that someone loved me.
PDD-NOS seems to be a big root of my anxiety and OCD. I do not understand people. The fact that people can form new friendships is fascinating to me - I never understood that magic that occurred between two people to make that happen. The idea that some things - jokes, reading, some statements, appropriate topics - are not acceptable to some people or situations never occurred to me when it did. When it finally did, my obsession went into overdrive, constantly trying to analyze (objectively) where and how and why humans work. I cannot have a "natural" reaction 90% of the time. I've learned by watching people and their reactions to me and others what I should say and not say, and I still screw up. I don't know how people know instantly how to interact together, without thought and analyzing of the possibilities. It seems like literal, impossible to reach magic.