I know exactly how you feel, I do most of my riding alone and I've turned back on a ride (once) and felt the same dejection. I certainly wouldn't classify 15-17 mph a social or beginners ride.

Cycling can be a very lonely sport.

If you know that many people who would be interested in a social ride you could make the first ride a short one. Make sure everyone knows the route and let it be known that you'll wait at the top of a hill or at an intersection to make sure everyone is there. Be prepared to go back and look for a rider if they fail to show in a reasonable amount of time.

If this was a club ride, see if the LBS has rides. And vice versa.

Don't give up on group rides and don't be too hard on yourself.
I think it was a combination of false advertising and less than great ride leadership.