WHAT IS THE FOG?
It is difficult to understand, and even moreso to get a grip onto the reality of it actually having happened to your WS/FWS.
There are some on the site who don't believe that it exists; in majority this is the group whose spouses have left them for the OP. Their POV is certainly understandable, given the fact that they've chosen to change their entire existence because of being in what we call the fog.
Most of the WS's on the site, however, agree that the fog is a very real place and state of mind.
There are three different kinds of "fog". WS fog, BS fog and OP fog.
THE WS FOG
The best way to describe the WS fog is to think of it as a type of being "brainwashed". The WS, when first involved in the A, feels complete thrill and excitement due to this sudden attention from someone new. It is a type of high that can only really be compared to a drug. The WS may feel "in love" with the OP, and the thrill to the WS is very real.
Soon he/she begins to convince themself that this A is good for them for whatever reason, in order to rationalize the guilty feelings that they feel. This is the beginning stages of "the fog". The WS may tell themselves that things have been bad in their M for a very long time, and the OP seems to be the only person who understands them. Many times, when a person is trying to rationalize something, they will tell themselves this rationalization over and over, causing them to actually believe what they are saying, whether it is real or not.
This constant inner dialogue of "The OP is the only one who understands me" and "Things are so bad in my M that I deserve some happiness with OP" causes them to lose all sense of what they used to know. They become psychologically addicted to the A, and need the constant high that it brings, many times with total disregard for the feelings of their confused BS.
After the A is found out by the BS, the WS very often has trouble giving up this fog. They may feel as if the OP is their soulmate, or that they have lost complete feelings for their BS, because they have now demonized their BS as someone who doesn't care for them anyway. When the A is found out, many times the A is still new to the WS, so those intense feelings only magnify for the WS. Feelings of "How dare you try to ruin my life?!" may surface for the WS towards the BS, because they have so completely brainwashed themselves. This is the WS fog.
HOW THE BS CAN DEAL WITH THE WS FOG
Dealing with a WS fog can be one of the most depressing and trying times in a BS's life. You are already dealing with the fact that your M has been polluted by an A, and now the person you married and thought you could trust has suddenly become some sort of pod person who seems to feel that their A is justified. Rest assured, IT IS NOT.
Although your WS is still in the fog, if he/she still loves you (and many times this will be followed with "but I'm no longer 'in love' with you") they may try to work things out. Both of you maintaining NC with the OP is essential to "wake up" your WS.
It is important that you sit down with your WS and have an adult, no screaming, taking turns conversation. Listen to the words that your WS is saying. Don't just listen so that you know what your next argument will be, but *really* listen. Some of what your WS says will be valid, and you have to accept that, although you were not the *cause* of their A, you did contribute to some of the problems in the marriage. Some of what your WS says will be completely the fog talking. This is no time for beating yourself up over "what you should have been doing". This is the time where you listen, figure out what you could do to improve the M, and what your WS needs to do (aside from maintaining NC and being an open book to you) to improve the M.
Validate your WS in the points that are accurate on your part as to not being the perfect spouse. However, hold fast to the inaccuracies that your WS points out, because as it happens so often in life, your WS may be projecting what he/she dislikes about him/herself onto you. Make a list of what your WS considers as marital problems, and down one side of the paper write what is accurate and down the other write what is inaccurate. You may be able to list examples of the inaccuracies that your WS has projected onto you about your WS!
Show your WS these examples and inaccuracies. It may not help right away, but it may just stick with your WS over time.
Most importantly, work on yourself to improve the M. In a best case scenario, your WS will see what a fool he/she has been, come 180 degrees out of the fog, and become remourseful, and will continue to work on the M all the rest of the days of their life.
THE OP FOG
The OP will occasionally experience a fog as well. Sometimes he/she had no idea that the WS was married, in which case it really was not their fault that an A took place. Usually, however, the OP knew full well what they were getting into. The OP fog consists of brainwashing themselves into believing that they have a chance at "true love" with the WS, and that he/she was sent to the WS to "rescue" him/her from "the big, bad BS". Unfortunately, not much can be done by the BS to get the OP out of his/her fog, but the one thing a BS can do is CONTROL YOURSELF. Do not give the OP any kind of satisfaction by letting him/her know that you are bothered by anything they do. Simply take the high road. As is said over and over on the site, the best revenge is living well.
THE BS FOG
The BS can also experience a fog of their own. We usually refer to this fog as "denial". This can be in reference to denial that the WS would ever have done something like this, so when you've caught the WS and the OP coming out of your bedroom together, of course your WS was just showing the OP a picture you've had on the wall in there for years. In basic terms, the BS is any kind of fog regarding the WS.
OVERCOMING THE BS FOG
Only you and your WS can help you to overcome your own fog. Denial is best overcome when you are slapped in the face with reality. As cliche' as it sounds, "the first step is admitting you have a problem." The BS fog is nowhere near as intense as the WS fog, and can much more easily be overcome. But, as with any problem, the solution lies only within you, yourself.