Yea sounds like karma man. Seriously maybe it's for the best though. How much could you have loved your fiance if you are willing to cheat on her? Also how unfair is it to your fiance if she is marrying a guy that will cheat on her? She deserves better.
Although I don't like the way you try and blame it all on the 19year old =/
''I'm not saying I'm a victim here ------ I learned who was really responsible for my woes. You'd think I would be smarter than that, since it's my job to spot bullshit and all. I'm sure she's had many good laughs over me.''
Sounds like you are saying the entire thing is her fault. You have to own up and accept when you make mistakes.
My advice would be to consider if you can actually do a monogamus relationship. Maybe it's just not for you. So find a girlfriend that is willing to have a non-exclusive relationship. Or try and find a girl that you love enough that you simply aren't willing to cheat on her.