This Sounds Stage Fright Related
The initial description of this sounded like stage fright, and the comments thread (which may get edited into the question) confirmed it, so I’m going to answer on that basis.
The bad thing about stage fright is stage fright: Believe me, I have been there. My high school forced all its students to take a speech class, and that entire semester– something like 12 to 14 speeches– I’d end a five minute speech just drenched in flop sweat, knees shaking, practically vibrating in involuntary panic, and experiencing almost out-of-body levels of fear.
The good thing about stage fright is, it does get better with practice and exposure. Maybe not fast enough to suit those of us so afflicted, but it does. (My university program forced another speech class on me, and many classes had a public speaking component. I got lots of practice whether I wanted it or not.)
If this really is a stage fright or just general nerves-related issue, then you actually asked two closely related, but separate questions:
Can you recover from this? Of course! If you just mean an awkward first session amongst (presumably) friends and friendly types, then yes– you just say, “Hey, guys, sorry bout last session but my nerves got to me. Bear with me a while, as I get used to this.” After all, they didn’t exactly step up and volunteer, did they? No, they drafted you!
How do you stop freezing up? Well, there is no magic, here. It’s just exposure and practice. Your instincts are already good– you’re familiarizing yourself with the campaign material, taking notes, generally preparing, etc. With very high likelihood, the more you do this, the better able you will be to function your way through it. And the more you can do that (usually) the less severe it will be the next time. You can read through any stage fright self-help articles you like to find what resonates with your experience the most, but the common theme in all of them is, “Don’t give up.”
One particular thing that worked for me, which might work for you here is the notion of the Friendly Audience. Even in general, most of us will never have to give a speech to a truly hostile crowd. Most people who attend a talk are at worst indifferent, and usually start with a baseline of sympathy and support for the speaker. But this isn’t even that– this is gaming, among (again, presumably) your friends. These people should actively want you to succeed (because if you do, they will have more fun) and should not be that interested in raking you over the coals. Moreover, it’s gaming and not a structured, formal presentation. It’s okay to take a minute for a deep breath. It’s okay to take a minute to check your notes. It’s okay to structure a 5 minute break every hour if that’s what you need. All of that is okay.
If this is important to you, then my boiled down advice is: (1) Don’t give up, and (2) remember that this is (or should be!) the very definition of a friendly audience that is fundamentally on your side, and (3) don’t give up.