Wow. Yeah. Don't beat yourself up!
You did great - got separated by traffic and fell off the back. So what?
Next week, circumstances may favor you and you'll do awesome. Sounds like the group leader was encouraging you. I wouldn't be so hard on him.
If the B ride is not challenging you and you want the challenge, then yeah - I'd say give it another try. The first time I tried a group ride, I got dropped so badly I wanted to cry. The LBS owner's wife escorted me back to the shop 20 minutes after everyone else got back. I didn't know the roads. I would have been totally lost without her. But, stick with it. You'll find your group on the ride and settle in. Good luck!