Sounds like you might have done all that you could. Sometimes the AI comes at you with so much overwhelming force that you cannot win.

The most effective tactic IMO is the one you tried: mass your cavalry with your general and use it to chain rout enemy units. Keep hitting them in the rear while your other forces engage in the front. Once one enemy unit breaks, nearby ones are much more likely to crumble. I did this once in exactly the same kind of situation you describe and it was exhilarating (and worked), but it was touch and go. The important thing is to try to stop your cavalry being mangled - keep them moving and pull them out of a melee if it looks like you can't break the enemy unit without heavy losses; and charge another (or re-charge the same) instead. It can be important to pursue routing units so they don't rally but often you just can't spare the men.

A much more lame trick is just keep your general running away from the enemy. Concentrate on killing their cavalry and then they can't catch you, so as a defender you win. But that's an exploit and I really should not do it (in STW and MTW, you would auto-rout in such a situation).