I really have looked at my role in this marriage and want to take away some lessons for future relationships. Like you say, its trial and error, and if you don't learn from your mistakes you are bound to repeat them. I was not looking for a young 23 y/o when I met my wife. In fact, I thought someone that age might be trouble and a few of my friends said don't do it based solely on her age - they thought she was still at the partying stage. Instead, a few older women I knew who knew me and my wife and her family thought we were a good fit. My wife was cooking meals like casseroles for me when we first started dating and already had a job. She was sewing buttons on my clothes and was really nurturing. She seemed mature. I think emotionally though she is very immature. I always worried a bit that she had not dated many guys before me. But she was so damn reliable and loyal and loving for our first three years of dating that all doubts were gone. My wife is the type of woman who goes to a bar or pool and all eyes are on her. But her eyes and attention were always on me - until last Fall apparently. I really don't think I could have predicted any of this and in some ways that makes me feel better. I know I didn't contribute to the A. I am not perfect but many of the flaws I read about on this board and others I simply don't have. I'll be honest - this thread has been very uplifting for me. It's one of the reasons I keep posting.
Clemson, sorry you find yourself here. I think you're handling things really well under the circumstances, for what it's worth. I'm posting to hopefully give you a bit of insight into "WTF just happened?" and reinforce your belief that it's something that couldn't be predicted.
You, of course, know you wife best. We've had posts that talk your wife's intellect and as you've mentioned her emotional immaturity. To expand on the second point:
There is a mindset, that I think is more prevalent in women than men, that if you're in love with your spouse you won't find other people attractive.
A good friend of mine stated something similar over dinner. My response to her was that I live in a big city with plenty of attractive people, that initial attraction is not something I can control and I wouldn't use it as a barometer of the health of my relationship. I think your wife had the same mindset as my friend.
What's less well known is that women in their early thirties have a spike in their testosterone levels and this can lead in some women to feeling unsatisfied in their life or relationship without really knowing why.
An exert from Women's Infidelity:
Infidelity, Cheating Wives - Women's Infidelity
Women at Stage 2 experience reawakened desire stimulated by an encounter outside the marital relationship. Whether these encounters with a "new" man involves sex or remain platonic, women will typically give a tremendous amount of emotional significance to these encounters.
This is what I think happened to your stbxw and explains how she so rapidly replaced you with someone she had only met for a few days. Having that "no eyes for anyone else because I love my partner" mindset + being dissatisfied with her life but not knowing why + the attraction to the new man + testosterone not only making her feel dissatisfied but making sure she did something about it. That's not to give her an out; we're all subject to biological forces and drives but we can override them.
Hope this helps and good luck on your journey.