Hmmm..I think the 6 week secondment is a good move, but given that this has been rumbling on for years, I don't think that an absence of that duration in itself will fix this.
In all affairs, attached parties need to justify to themselves why they are doing this. Very often, the justification they provide to the affair partner is not the same as the one they are telling themselves and even then, there is often a dissonance between what they are telling themselves and the truth. The affair partner is therefore often 2 whole steps away from the truth.
So, he was telling you that he was staying because he loved his DC and they were a family and implying that he had settled for "good enough" in his marriage. He was also telling you that he loved you.
He might have been telling himself that he loved his wife, had some feelings for you and that this was a fairly low-risk affair, especially when you were married and had just as much to lose as he. You have proven over the years that you're never going to rock the boat for him or cause hassle or difficulty. So for him, this represented a low-risk opportunity to have a relatively undemanding affair. He would also have been comforting himself with the fact that he made no promises to you and so can't be "blamed" by you.
The truth on the other hand might be very different. His connection to his wife is very strong and he simply cannot imagine life without her. He loves her and would stay with her, whether there were children still at home or not. But he is a selfish man who feels entitled to a distraction every few years and since he is ultimately lazy, resuming things with you doesn't require much effort, since chances are, he will meet up with you every few years. When he does, you feel familiar and warm and he says "why not?" He gets all the good feelings of being adored again, different sex for a while and no demands.
The massive clue to this man's true feelings for you is that this last time, he should have realised that you were in a terribly vulnerable place, looking for a distraction yourself from your grief. If he had genuine feelings for you, he should have stepped away. Instead, he chose like you did, to resume an affair that is not going anywhere and is actually bringing you more misery on top of your grief.
He doesn't love you enough to let you go.
This man is also deluding himself that since he has been "honest" about your future together and has never disparaged his wife, he is still a decent sort. I agree with the poster who said that you in turn have deluded yourself that if you don't hear about his marriage and his wife, you can pretend in part that they don't exist. But you have perhaps been deluding yourself that this was an honourable thing to do - not discussing his wife and marriage.
His wife and children need to be humanised for you. You really need to step in that woman's shoes and imagine how it might feel. She has been robbed of her choices for years through this deception. She has been suffering unimaginable hurts by her husband sharing his intimacy. Every hour spent with you is an hour taken away from that family.
Now unless he is a serial philanderer in between bouts with you, that woman will have noticed a change in him whenever it is back on with you. She might not be able to put her finger on what it is this time, but there will be an air of familiarity to how he is behaving. If she asks out of concern, he might tell her he is stressed at work. But she is suffering - and don't ever delude yourself that she isn't, or hasn't, over the years.
Take the advice about how to detach from this man and ask him to honour it too. Tell him you hope he makes it up to his wife and finally treats her with some dignity and respect. Since he is at heart a moral coward though, I doubt he'd ever tell her the truth about her life for the past 10 years. That is her misfortune, but I hope that karma will catch up with him in the end.
Use the 6 weeks to detach and then get that counselling. It is vital that you are honest this time with the counsellor. Get to the bottom of why you have been drawn into deceit over such a long period of time. Resolve never to be that person again.