I agree--try riding by yourself at least some of the time. You can find your own pace and you don't have to compare yourself to others. I find I much prefer riding by myself (at least, on my rail-trail), because I'm a little competitive and self-critical and I keep finding faults with myself because I see what other people do. Riding needs to be fun.

I'll echo what everyone else has said--sometimes it's you, sometimes it's the bike. I don't climb well. Why? My bike doesn't quite fit me and is kind of heavy...and I'm not in the best of shape, or particularly light.