I cannot speak for the other former waywards of this site and I am sure there is plenty information in The Healing Library on this.
The biggest question from a BS is WHY?
Why did you betray me?
Why didn't you just talk to me?
Did you think how badly you would be hurting our family
The answer to the WHY lies strictly with the WS.
The BS in no form or fashion responsible for an affair. The marriage may have had issues, but none justify an affair.
The WS may tell you they were unhappy for a long time.
That they love you, but not in love with you.
That we grew apart.
You were never there emotionally for me
You were never home, even when you were.
Those are reasons for marriage and individual counseling, but no reason for an affair.
My becoming a WS was due to major conflict avoidance and being passive aggressive. Instead of facing an issue head on, I would do other subtle things.
Get silent or leave the house and do something selfish like hanging out by myself.
Instead of telling my wife I had an issue, I would just blow it off and see if she would notice and fix it.
9 times out of 10 she never knew I had an issue. She couldn't know, I didn't say anything.
Like I felt she didn't want to be around me.
This was not true, if you do not make yourself someone people want to be around, then they really don't want to be around you.
You can give off a negative vibe and people can feel that sometimes.
My wife just thought I was having a bad day. She had no clue I was feeling that way about her.
Eventually this will lead to resentment and entitlement and you feel that you are entitled to attention. You feel unappreciated. These are all coming from YOU, not your BS.
You open that door in your heart to a person that either knows your situation or you reveal it to them.
Most of the time with your perspective.
Most of the time with your own spin.
Things that you have created in your own mind that are not true.
Poor boundaries,selfishness and even disorders play a part, but still are not excuses.
To a full blown wayward, they have to demonize the BS. Make everyone think that they have a right to go outside of the marriage.
A wayward may even know that they are wrong, but have to convince themselves that they are entitled to do and behave the way they are.
Also having developed "feelings" for the OP clouds the mind of the wayward to make them further believe they are justified.
Not being in the right frame of mind to know if you invest feelings into someone other than your spouse, then you will most likely fall for them.
The wayward mind is full of fantasies and made up scenarios, also along with "feelings" for the OP, that it becomes hard to really see the damage they are causing and what their "real life" is.
However once the third person is removed and the wayward can really see how much their spouse cares for them, "real life" starts to become more visible.
The wayward mindset is unconventional, irrational and irresponsible.
It takes unconventional methods, rational thinking and true love and endurance for a BS to get them back on track.
Most of the time a wayward will snap back once they realize they are about to lose their spouse.
Because most of the time, a wayward really does not want to leave their spouse nor do they want their BS to leave them. They think they do, but they really don't most of the time.
Only after they can see what they are really about to lose, especially if the BS was a really good spouse, will they start to leave the wayward mindset and take a look at real life around them.
That and NC (which is first and foremost) will be key tools in fighting the wayward mindset.
Just my thoughts. I'm sure others have more valuable information.