mprince

Build up your base through solo rides (varying intensity and intervals) as many have said, and perhaps find a slower group to ride with for a while. While getting spit out the back of your current group ride isn't fun for you, it also isn't fun for the group as they slow down to get you back on. Part of the challenge is to find a group that fits where you are, you can always move up to a more challenging group when your fitness/endurance is there.

Group rides can be tough, I used to do a ton of them. Today most of my riding is by myself and at least for me, my enjoyment of cycling has increased dramatically as a result. I've lost the social aspect of riding my bike, as I learned that a group doesn't really add anything to my time on the bike. This aspect is different for everyone, as we all have different motivators to get out there and ride and different goals while we are doing it.