How you "think" of OM is one of the most bedeviling questions of this whole experience. Obviously, the less you think of him at all, the better. Your temptation (as mine has been) is to think of him as a replica of you, as fulfilling the same role for your W that you do (or you hope to once again). But its just not like that.
What is it like? I have no idea. My W has acted as if he doesnt exist, even though I have plainly stated that I know he does. When I mentioned him in our conversations after BD, she would play him down as if he factored into her decision not a whit. When I told her she was having an A, and intending to stay in our house, she told me she wished I wouldnt call it an affair.
All I can think of is its some kind of short circuit in the WAS' brain.
As for your question about what this forum is about: The practice of imagining your life without your spouse has two benefits, making you work on yourself, and also possibly making you look like a better option to the WAS. Its all very counter-intuitive, and most of us struggle with it from day to day, which is why we are here!
M: 33 W: 33 M: 9 T: 10 3 S's: 8, 6 and 1.5 BD: 8/3/14 Living together