As others have said, as a front-line fighter with a two handed weapon, expect to get hit. That said, AC 20 is rather weak. As others have said, you need to talk to your DM about the equipment level too).
So, what can you do? Here's where I'll differ from the other posters.
1. Use your mount. Paladins get mounts for a reason and it's not so they can sit in Celestia doing nothing. If you have ride by attack or spirited charge, you'll increase your damage and hopefully decrease the amount of time your opponents live to damage you. You can also make the DC 15 ride check to take cover on the side of your mount when you're not attacking (less useful in 3.0 than 3.5 since attacks could hit cover then).
2. Don't fight defensively a lot. Combat Expertise and fighting defensively are all right but if you just go from being hit 80% of the time to being hit 45% of the time but go from hitting 90% of the time to hitting 45% of the time, you're not usually winning in that exchange--especially since at 9th level, you often face foes with abilities like regeneration and fast healing so if you're not damaging them quickly and consistently, you're not (effectively) damaging them at all.
3. Instead, up your offense. If you're a 9th level human paladin you've got 5 feats. If they're Power Attack, Divine Might, Improved Crit, Mounted Combat and Ride By Attack, you're doing fairly well. If you don't want to use your mount a lot, you can trade the mounted combat feats for cleave and weapon focus or something else.
4. If you can't get your party to buff you (which they should if they want you to protect them--magic circle vs. evil, 3.0 False Life (unfortunately, the 3.5 version is Personal), Magic Vestment, Barkskin, etc), you'll get hurt more. But there are still some things you can do for yourself. As a 9th level paladin, you've got first and second level spells. Protection From Evil will help you out. So will Resist Energy (2nd level spell--it sounds like you run into the odd fireball here and there). Scrolls (375gp for the relatively long-lasting Magic Circle vs. Evil) or Pearls of Power can help here.
However, what would help the most would be a Displacement spell from your party members.
5. Realize that your hit points are your primary defense. Even if you had the full cash alotted to characters of your level, you'd still get hit quite a bit. So get yourself an amulet of health.
6. Maximize your defenses for the money you have. +2 fullplate and gloves of dex +2 (for the 10-11 dex paladin) are cheaper than +3 fullplate. They give you the same AC but the former combo saves you 1000gp and gives you +1 reflex, +1 to dex skills, and +1 init. And, if you can get your party to buff you, remember that enhancement bonusses can be substituted for with Magic Vestment and Greater Magic Weapon. Sure, you're vulnerable to dispel magic, but if you're a high level character in the poorhouse, you're going to have a lot of vulnerabilities. You might as well reduce them as much as you can.
The cheapest way to get AC would probably be Ring of Protection +1, Amulet of Natural Armor +1, Gloves of Dex +2, +2 fullplate (or just go for mithral or adamantine fullplate and beg Magic Vestment off the cleric). Total cost: about 13,500gp. Total AC (assuming starting dex of 10): 23. Not too much better than 20 but not too bad either. However, long term, an amulet of health is probably better than natural armor, and Magic Circle Vs. Evil/ Protection from Evil are cheaper than a ring of protection so you can spend that money elsewhere.
Under 3.0, look into buying Pearls of Power II and Lesser Rods of Extend Spell and begging ability enhancing spells off other party members. (As a Pal 9, you might be able to talk your DM into using the 3.5 spell list which has Bull's Strength on it). Sure, you're vulnerable to dispel magic and you probably still won't have your statboosts when you're in your down time but if you're short on money, something's got to give. Statboost items are an area you can skimp on in 3.0 by delaying their purchase until you can afford +4 ones and making up for the difference with spells.
If you can get long term buffs off of party members (provided you give them the slot back with a pearl of power), you could do something like this: +1 fullplate, +1 falchion, Lesser Rod of Extend Spell, Pearl of Power II x2 plus odd change and still get +3 armor and weapon through magic vestment along with +2-+5 str and con for 18 hours. Your +3 weapon (GMW) would only last 9 hours but that would still cover most encounters during the day.
7. Look into getting a Ring of Blinking (25kgp) in the future. 20% miss chance is a small price to pay for giving most foes a 50% miss chance, taking half damage from area effect spells, being able to walk through walls and out of grapples, etc.
8. If you want to keep the concept, you could always switch your weapon for a scimitar and wear a buckler for when you need the AC. (It's only -1 to hit for wielding your weapon 2 handed with a buckler).
And if all else fails, start over and come in with a dual wielding quarterstaff expert Vow of Poverty Divine Might paladin. (And force the party cleric to cast Spikes on your quarterstaff every day).
