Loves to walk - I'm afraid you are absolutely right - and I'm sorry. This is a horrible phase of affairs, but absolutely textbook, as others say.
This is what I think has happened.
Everything was fine in your marriage - usual stress and strains connected with raising children and building careers etc. but you might tell us that everything seemed fine. You would have said that your H was a good man - although perhaps a bit selfish and lazy - but you had no doubt that he loved you to bits and that you loved him.
Then he meets someone at work and feels something. He marvels that unlike you, she loves football, which is a passion in his life.
He starts to look forward to their conversations - she really seems to "get him". But he doesn't worry that this might "mean something" - after all, nothing's happened and it's not going to. They start to see eachother outside of work, but hey it's football, so it's all fine and in any case, you know about these trips and seem cool about it.
Then they have a few drinks after work and he's suddenly got to confront that his feelings are more than just friendship. There may or may not have been a drunken snog. This terrifies him and he blurts it out to you.
If at this tipping point, he had found some way of severing the friendship, so that this heady intoxication was starved of oxygen, there would have been hope. If you had realised at the time what this really meant - and taken it more seriously and insisted he confronted it properly, there would have been hope. Please though, don't beat yourself up about this - these lessons are always best learned in hindsight and the main responsibility was his.
Instead, he found himself thinking about his friend in a new light. A fundamentally decent bloke, he realises at a conscious level that he has no justification whatsoever for an affair - he's happy with you and he doesn't want to leave you. He loves his kids.
However, at a very subconscious level, he starts to blame you for being an obstacle to something that would feel good. He works hard, doesn't he? This friendship is a beacon of light and fun in a life filled with mundanity and responsibility.
So, in order to give himself permission to cross the line, he starts to reduce your marriage from what it once was. He starts to distance himself from you. He stops making the sort of loving, kind gestures that you've become used to. He starts to "hide away" more, at the gym, in the study on the laptop, at the football [or reading about the football). He starts to become more selfish and lazy - and you react to this.
Instead of accepting responsibility for what is bad behaviour, he counter-attacks and makes you feel like a nag - and as if it's wholly unreasonable to challenge his behaviour. You feel like you're going a bit mad. This isn't conscious behaviour on his part, either. When he looks back, he will be horrified when he realises this was what he was doing.
Eventually - and this might be driven by his friend to a great extent [he still can't allow himself to be the "deciding" adulterer) he submits to an affair. He feels alive and as though all his senses are heightened. But because he's basically a good person, he's crap at compartmentalising the two lives and feels guilt. If you're nice and kind to him, this makes him feel worse, so he ups the ante and behaves in a totally foul manner, to prevent you from loving him. You react with hostility and contempt - and that suits him just fine because he's got the justification to continue with something that makes him feel good.
The deciding factors about where it goes from this point onwards are: his feelings for her, his feelings for you - and the point of your discovery.
If the intoxication with her starts to fade, he will start to see things in her that he overlooked before - the rose-coloured glasses come off and she starts to become an irritant and a problem in his life that he cannot resolve easily. He starts to worry that perhaps this won't end without fallout of some description, and this terrifies him. He starts to display signs of stress. At the same time, he starts to see your good qualities that he had been overlooking for such a long time. He starts the journey back to you. If discovery happens at this point, the prognosis is very good for recovery, as long as he works on himself first.
It's going to be easier if your discovery happens before sex has taken place. Once men consumnate an emotional affair, their responses are much the same as women's in the same situation - it deepens feelings [for only a short time sometimes) and for men, this is the point when they can no longer pretend this is a safe friendship - so the guilt really starts to kick in and their behaviour with their spouses gets markedly worse.
If he's convinced himself that he is "in love" with her, this is the most difficult point [for you and your marriage) for discovery. He has to make a decision at this point - and he will blame you for that.
I would say looking at your post that he is either just at the point of consumnating this relationship, or it's either been going on a for a short while. I don't think it's been going on for the whole six months, FWIW.
If I'm right, you've got to find incontrovertible evidence of what's going on and make your decision based on that. Do not make your decision based on what he's telling you. If he's in an affair that he wants to continue, he will deny it - and if he hasn't yet started one, he will deny that he ever would have crossed the line. He might even believe that himself, but it would be a lie.
To move on, he's got to sever this friendship completely. That must be his very first action. He's then got to start a painful process of working on himself, but that process depends hugely on whether this is pre-affair or mid affair.
You are not going mad - you are absolutely right and trust your instincts above all others. Do not under any circumstances go away next week without him. If you provide the circumstances for what is now an emotional affair, to become a sexual one, your marriage will suffer more. I'm not saying it won't be recoverable, but as I said earlier, sex and emotions are an even headier mix than one or the other.
I would spend the next few hours finding out information, working out what stage this affair is at and then coming to a decision about what to do next, in terms of confronting. He will only admit to what you can prove. If you think it's not sexual yet, I'd start clearing diaries to ensure you spend a block of time together so that you talk this out till you're hoarse. If someone can replace you on that holiday, I would stay home. The kids can have a great half term and you get uninterrupted time.
For this to have happened, do not fall into the trap of thinking it must be the fault of the relationship, or even you. It's very common and it has nothing to do with you, or even your marriage. It is about him.
Take some comfort that perfectly good people can have an affair - friendships can cross the line all the time, but men in particular are useless at realising where that line is and won't admit what they are involved in until things become sexual. Based on the little you've said, there are signs of hope that this man does love you and doesn't want to hurt you, but he is horribly conflicted and in a haze of denial. It is only when everything is out in the open that the denial starts to fade.