I have the same tendency. For my first few rides this year, I burned myself out on the first half of my standard loop and my average speed suffered (16-17 mph). Then I did a ride with my friend who just started riding, and I pulled him most of the way. I made sure to pace myself at the beginning so I didn't lose him, and surprise! I hit the end of the ride and our average was 17.7 mph. Almost a full mph faster than my best this year on that loop. (Although still 1 mph off the end of last year's pace.)