I think Dan's advice for MMM is great!
MMM, you're "achingly unsure of whether or not I made the right choice" of sex act, Maybe your girlfriend feels that way every night! You're a sexual power vacuum, and she's filling it--which she may very well love, or love most of the time, but on at least a few occasions she's let you know she wants a break.
She's not asking (necessarily) for some amazing sex fantasy she's never heard before, I think she just wants you to pick the position every so often, because she does it all the other times. (Who else did you think was doing it? Did you think there was like a sidebar on slate.com
that she checked every day? Did you think it just happened "organically"?)
Anyways, I sounded mean but I didn't mean to be. You're doing a-ok! You don't need to be less boring, just remember that picking the position is a job that not everyone like, because, case in point, you don't like it. So maybe she doesn't either, and maybe you can figure out how to share it.