Agree wholeheartedly with Madison--only time in the saddle, and focused training will make you faster and improve your skills. Getting new riding partners is also a good idea if you want to improve your speed to keep up with the others. Do you WANT to be fast or are there other things about riding that you enjoy? It doesn't sound like the current situation is making you happy.

There's nothing wrong with being the caboose, in my opinion. I find that being last in a group of fast riders has its advantages, as others have already pointed out. The important thing is that you are out there. Focus on enjoying yourself and try not to worry about the others.

One of the reasons that I prefer to ride alone is that I can do my own thing and don't have to worry about adjusting my speed, route, etc. to suit others. Some days I feel strong and can hammer, but there are days when I just want to ride for smiles, not miles.