Kimmy,
Stop beating yourself up for just one ride. Just like you I always ride with B+/B and I believe we even have the same Vita bike Few weeks ago I decided to go and ride with A- group of very strong riders. Let me just tell you it was harder than my first (and only one so far) century for me. I felt like I was just trying my hardes to keep up while other were pedalling very comfortably. It happens to most of us and this is the only way to challenge yourself. Would you really feel good about yourself if you never tried it out in the first place or would you always wonder "can I do it?". So what you couldn't keep up for the rest of the ride, next time you will go there and do it better and you will be stronger. 14.7mph average that comes from your computer is very misleading, all the slowdowns, stops count in too.
Anyway, I am pretty sure that next time you attempt to do it you will feel better!
Good luck
FN