Funnily enough - see if you can get ahold of an item of 'reduce person'
Surprisingly, being reduced isn't going to hurt you as a fighter (small) vs. a fighter (medium) _if_ you focus on your critical hits (keen falchion, imp crit, etc. whatever you can do).
Falchion at 18-20 will go to 16-20 with any one of those (lower in 3.0), assuming 3.5 rules.
Reduced fighter gains +2 Dex and +1 AC for a total of +2 AC and +1 to hit with weapon finesse weapons.
That's one option.
Second option - plate mail is nice armor - horrible to your dex. If you have a decent dex modifier, go with something lighter. Quicker speed, lower check penalty, and more dex to your AC.
Third option:
Forget plate mail entirely (ac 8) and use items of mage armor and the like - you can match the AC of plate mail, or better it, fairly easily (as mentioned, easier if you have spell casting buddies)
Fourth option:
Use tactics. You can legally advance to the first threatened square and ready an action to hit something if it hits you (this would then include both the weapon or the creature when triggered). After the first such action goes off, ready an action to hit creature and 5' adjust. This works best with reach weapons, of course, but similar variations with swords also work.
Other tips:
Use cover/concealment. Force that miss chance vs you. Buy incense, carry lamp, smoke the area and force miss chances. An item of darkness works great in 3.5 if you are an elf, or the like. hide behind trees, bushes, rocks, other characters, whatever works.
At 9th level with an AC of 20, you are going to get hit. Period. Any fighter of 9th level will have a +9-+18 to hit range. That'll hit you 50% to 90% of the time. If you 'accept' that and build a wrestler type - let 'em hit, then grapple.
A diffferent concept - look up Karmic Strike in the Complete Warrior. If they hit you, you hit them as an AoO. You can trip or grapple on an AoO. With CR, you could trip them on a hit, trip them when they stand etc.
Personally, I'd dual wield Falchions with Imp Crit, 2 wpn fighting, imp 2 won fighting, 2 wpn style, 2 wpn defense (FRCS feats), combat expertise, and an item of reduce person.
Assuming plate + no dex or even chain shirt with +4 dex, you can hit 24-25 AC fighting defensively, or better. With imp crit and a 16-20 crit range on your 4 attacks, you stand decent odds of doing similar damage as you are currently. Possibly better if your GM will allow Wpn Finesse on Falchion...
Take one level of rogue and bluff, or max out intimidate and shake your opponents to get either SA +1d6 or +2 AC (shaken = -2 to attacks and saves) vs a single target.
IMO anyway