Ouch.
04chryslr, you need to be honest with both ladies. Tell the x that it was a big mistake. Tell the fiancee that you made a big mistake.
I have to agree with some of the others, your fiancee may forgive you (if you are lucky), but she will NEVER forget it or let you forget it.
If you are forgiven, do not fall victim to the 'once a cheater, always a cheater' mantra.

BTW, I was engaged at age 21, married at age 24, and been I've been married for almost 6 years.