Talking to a psychiatrist isn't quite like talking to other doctors or "officials". It's not about "admitting" to anything. He probably mostly wants to know about your pain and its causes. If he asks about drinking or drugs, just an offhand, "Some, back in college, umteeump years ago" is sufficient. If he asks more details, be specific but don't be detailed (don't tell whole stories, just facts). Being factual about things you don't think germane or that you don't particularly want to talk about should satisfy what they ask without making them "curious" LOL. It can be hard with a psychiatrist.

My father had a "joke" about psychiatrists (he didn't like them but never really had much to do with them, my mother did/had to back in the 1940's for a brain tumor which is why he didn't like them; of course they couldn't help but back then, no one knew that yet) that went: A guy is walking down the hall and greets another guy, a psychiatrist, with "Good Morning!" The psychiatrist responds, "Why did you say that?"

If you can, get the psychiatrist on your side, talking as if you know what you are doing :-) "I have RSDS but don't think my pain doctor is very familiar with it and how much pain it can cause, can you help me think of better ways to communicate with him my needs?"