I did this exact thing a couple weeks on my group ride. I was actually somewhere in the middle, but thought I was completely left behind. I got discouraged and turned around. It turns out I wasn't far from the turnaround point for the whole group, and the rest of the group (which was behind me, by a few miles). But I felt alone and isolated, so that was it.
I am VERY embarassed about what a slow climber I am, and I ride with a really fast group. So it is kinda a double whammy. I just keep working on letting all of that go and focusing on MY ride. Since I have been riding with the group my average MPH has gone up about 3 MPH. It is getting better all the time, but I don't think I will ever be a particularly fast climber.
Trek Madone - 5.5 -Brooks B-17
Trek 2.1 WSD - Brooks - B-17 - Trainer bike;
Gary Fisher - Tassajara (MTB) - Specialized Ariel