Comfort. And it begins inside with your attitude. Get used to the idea that you don't want her to strip off everything. Cause lust can be a bottomless pit, if you just keep going for what you think you want, you won't appreciate it when you get it. Because you didn't make the most of what you had in the first place. Sex works like that - you can dive quickly to an orgasm, but that wouldn't be as good as if you had fun with it along the way, and got teased and prolonged it.
Also - you're getting a strip tease from a girl, and not just that, one who hasn't done anything with a guy she hasn't been dating - so you're pretty lucky here - regardless of what you rate her as.
So be appreciative of what she's doing, and make the most of it. That attitude of not trying to encourage her to do more will make her more comfortable, and possibly then, she might want to go further
But if you are thinking about that instead of appreciating what you've got right now in the moment, that probably won't occour! So the key is to just appreciate and make the most of what you have.