My Paladin has fallen. How can I restore him back to his old self?

Ehm...don't think you can. Once you've fallen that's it.

I have heard something along the lines of this:
increase your reputation, then undertake the quests given by the Order of the Radiant Heart.

If you've fallen from grace, unfortunatly that's it. Now your character is basically a fighter with no specialization opportunities. You can still persue the Radiant Heart Quests, but they will not restore your paladinhood.

BTW, if this were P&P, you would be able to atone for your fall from grace through your home church by having a high (I mean HIGH) level cleric cast Atonement and then perform some significant service for them (IE, slay Demogorgon, 12 Labours of Hercules, wipe out that nasty Lolth cult, you know menial tasks.)

Actually, according to 2nd edition rules, that is not totally true.

If you are tricked into doing something, whether by magic, stupidity, or whatever, you will become a Fallen Paladin, but can regain your powers through a significant act of service for the church (although trying to accomplish this usually results in your death).

This is what should have happened to your character is you are playing a Paladin and did the Windspar Hills area (which lead you to unknowingly kill a group of other Paladins). Unfortunately, the game still lets you be a Paladin, but oh well. You are instructed to kill Firkraag anyway to make up for it, which is what you should do to regain your Paladin status.

However, is you knowingly and willingly commit an evil act or change alignments(which differs because you can commit a bunch of non-lawful acts even though they may be "good"), that is it. There is no chance. You have sworn upon an oath and have willingly broken that oath. Regardless of what you do or how you conduct yourself from that point on, you cannot go back.

Of course all of the rules are subject tohow the DM plays anyway.