I think riding a lot does help and definitely trying to hang on with faster people. I get discouraged when I drop off the back a lot, especially if they decide not to wait for me.

Wind is the flatlander's alternative to hills. So you might consider that.
I think intervals are also good.

My average seems to be creeping up - from 13 mph two years ago to almost 15 now. Sometimes lately it has been higher- over 16, but I don't know if part of that is due to weather. Unfortunately, everyone else I ride with has been doing Carmichael Training spin classes for the last 3 months and they have all gotten faster than I have gotten faster, so I still can't keep up with them. Does that sentence make sense?

Keep riding. Push yourself when you can. And have fun.