Hi Malory. I'd like to say I have the solution to your problem but I don't. I'm just like you. Whenever I'm stressed, the anxiety tortures me with nausea , especially in the morning.
The thing that helped me the most when, 3 years ago, I was sick everyday, not being able to eat, was paroxetine. But, since then, I have been tappering it and I get the ocasional nausea. I had therapy and , one thing my therapist always said was that I shouldn't fight my nausea, rather learn to live with it - that would be the only way to get rid of her, not worrying about it. I know it sounds weird and impossible but, with time, it get's easier (at least, most of the time).

Regarding the date, been there too. Me and my boyfriend started going out a year ago (almost) but in my case, I didn't have your problem because to me it wasn't a date. We were already friends, just going out and things just started to happen. Still, when I realized the road I was going through , I panicked and that dreadful nausea attacked me. I just told him I wasn't feeling so well, due to the heat (it wasn't that strange since the weather was very hot).

I know telling you to stay calm is stupid (I hate when people tell me that) but, remember, it's a first date, he'll probablt be as nervous as you. Nobody's perfect. he doesn't know you never went out on a date or even kissed a guy and won't find out unless you want to and tell him (believe me) so, that shouldn't worry you. If you feel sick during the date, just go to the bathroom, get some fresh water on your wrists and neck and take deep breaths. Still, if you're not feeling well, just tell him. It will make the nausea immediately better. Say, for instance, that you haven't been feeling well since morning but you didn't want to cancel the date but you just wanted to tell him so that he knows why you're looking (or acting) that way. Getting it out, makes a big difference. I usually feel less sick (sometimes the sickness just goes away)

Well, that's what I know from my experience. Sorry, I can't help you more.