My wife and I got to 50 easily, we did not do much pvp, no space combat, and skipped the bonus series quests for planets. Chances are your skipping something, like the bonus parts of quest that pop up from time to time. We were even a level or two below the quests on Belsavis and caught up by the time we left the planet.
If your behind 3 to 4 levels, take the time to queue for pvp, or do space missions, go back and do a bonus series for a planet. It will help you.