Damn Val, that really hurts. Can you get one of those phone call tape recording devices? Get the lying chick on the phone alone -- have somebody you know call and ask for her -- about a Victoria's Secret catalog or something, then you get on the phone. When she's on the phone record the conversation and get her to say something that can work against her. It's not OK for court, but hell, for personal reasons, why not? You could even go so far as to tell her that when she told that lie it confused you because you'd always really wanted to be with her.
Here's an afterthought, people like this don't respond to how their actions are hurting others, they only respond to how much trouble they can avoid. Maybe... and I'm thinking this could be a kickass idea... you should get a civil attorney and sue her for defamation of character, slander and whatever else he can come up with. Whether you'd win or not is irrelevant, just the very act would be a very powerful statement. Hell if it did go to the court, she'd have to provide times, dates that you two were 'together', personal details, etc... she'd sink herself!
Call her and tell her that if she doesn't come clean you are going to take her for everything that she has, and that you know guys on the force that have wives that are CPS officers and you can influence them to come and take her kids away... scare the shit out of her... then back off ... get it all on tape (but be sure to dub over anything that could get you in trouble...) Get everything on tape, every commmunication -- she will say something that will be her undoing, you just need to be prepared for it...
Just remember, when things get really dark, just stop and say aloud to yourself "I am alive, I will survive." It sounds silly, but it really works.
Hang in there man, you've been through tougher things than this (or things just as tough) you will weather it, and you will always know what the truth is. What others accept is beyond you. They're failing to believe you is a testament to their weakness, not a reflection on your character.