There are several ways to "fall behind". Score, military, tech. Which are you talking about? Hard to tell from your post.
Notes
-- I think you might find that even when winning, it's difficult to "build enough military units". Best get used to that feeling.

-- research? Build Monasteries before you build anything else. Learn how to build specialized commerce cities. Learn how to effectively maximize cottages.
-- running low on cash? Who cares. Cash is a tool, not an end goal itself. Now, if you said "I never have enough cash to upgrade my units" then we could talk about solutions to having obsolete units. But, low cash
itself is not a problem.
-- score? Again, who cares. Score doesn't mean anything, except at 2050AD. In fact, having a low score might actually be a
benefit. I believe the AIs tend to take pity on you if you have a low score. They will tend to give you good relations, and won't invade even if they think they have better military. On the other hand, if you are top score, and they think they have better military, they'll invade at the drop of a hat, even if they are Friendly with you.
Wodan