--Not just for interviews, but for anything that you're freaking out about--
It's not that hard... okay, yeah it is, but it's not that [i]complicated[/i].
I have little to no power to determine whether an interview goes well or not, just like I have little to no power to determine if, say, I do well on the SAT's. Yeah, I can do a pre-test or whatever, and make sure that I get a good nights sleep beforehand, and eat in the morning, and do some basic stuff - but at the end of the day, my basic nature (as well as the basic nature of the interviewer) is going to determine how it goes. If the interviewer has good energy and whatnot, then I'll probably do well. If they don't, or if we otherwise don't sync up, then I probably won't get the job.
When you step outside of your comfort zone and into places laced with fear and trauma and whatnot, emotions just wreck you. They drag you down and quite frankly lie to you. But logic - logic is a comfort. Logic reminds you that the sick stories that your emotions are telling you are bullshit. Logically, you can do mock interviews with friends, and practice what you're going to say. You can try to get as much sleep as possible the night before, eat in the morning, and handle basics like that. But beyond that, the interview will go exactly how it'll go.
When I walk into an interview, or when I'm about to answer the phone for a phone interview, I have one single basic thing that I remind myself of. My entire interview strategy is centered completely on this one thing. Nothing else matters. All other techniques and pressures and things-that-I'm-supposed-to-say and everything else is worthless compared to this one thing. The thing that I remind myself of is: Let the truth be my guide. Let the truth be my safe place. Let the truth be my home base. That's it. It's as simple as that. I answer the questions with the truth, as best I understand it. No stress or pressure of mind-blank can stop you from knowing the truth. And yeah, there are a few "trouble-questions" that I have rehearsed answers to, or resume points that I have rehearsed explanations for. but beyond that, I just tell the truth.
Every action, every perception, everything comes from a place of emotion, logic, or both. With this kind of trauma-laced, outside-comfort-zone stuff, emotions are liars, and they never serve you - they only hurt you. They only strive for the status quo of unhealthy shit from the past.
The truth - whole or partial - doesn't have to be remembered. I can talk for minutes on end about whatever the interviewer asks, even if it's a question that no interviewer has asked me before, because it comes from a place of truth. Your doing this-or-that experience isn't something that you have to memorize - it's something that you deal with day after day. You KNOW the whole truth of what it's like to do this-or-that. You know the answer, without needing to memorize some stress-inducing thing that will just cause your mind to go blank if you try to regurgitate it on command.
"How am I going to remember to say ALL that? For EVERY question?" - you can't - it's impossible, so don't try. Stop treating it like it's something that you have to do perfectly - it's not. You just have to tell them how it actually is for you. That's easy, if you just let it be, and stop letting your stress and your emotions tell you what you're "supposed" to say.