loves2walk I've stayed away from your thread thus far, because I felt it was important that you heard from a range of other people who might have had a different "take" on your situation to me - and as you know, I contributed heavily on your last thread and still often wonder how things are.
I think what might help you is to articulate what is indisputable and not open to interpretation. It helped you last time to deal with what was after all, in the open domain.
He confessed an attraction to someone with whom he already had a close working relationship and with whom he had started to see outside of work, at drinks and at football matches. That person also admitted to him that she was similarly attracted. That would be a threatening situation for anyone in a monogamous relationship.
You well know that I think couples should discuss temptation and crushes, but what your H did instead was to drop this bombshell and then call you "pathetic" (original thread) and "ridiculous" for expressing disquiet and asking questions. Given that you both know his history of infidelity, even the most secure partner would have concerns and fears for this situation. Instead he unilaterally decided that the subject was not up for discussion. Even in your most recent discussion about the mutual acquaintance's marital breakdown and infidelity, his rebuke to you had the effect of putting a stop to any further conjecture.
Far beyond the issue of infidelity, the sense I get from what you write is that you are scared of expressing how you truly feel to your H. You've said before that he is better at winning arguments, will react angrily to any suggestion of mistrust and it seems that if you express what would be for most of us, perfectly normal worries and fears about a situation that he has created, you fear he will get angry and make you feel small again.
So what's indisputable? He admitted a mutually felt attraction to someone and offered you no reassurance or empathy. He belittled you for your insecurity. He continued to work alongside this woman and socialise with her at events that you cannot attend because of the physical pain you suffer standing at football. He started withdrawing from you, spending more time at the gym and at social events involving the colleague - and was guarded with his phone. He behaved horribly to you and the children for a long period of time. After yet another argument about a trivial incident in February, that resulted in him threatening to leave you and involved him pinning your DS against the wall, he apologised on the phone during the day, but instead of talking to you that evening as requested by you, he went to an evening meeting, sat staring at his phone with his head in his hands and then completely avoided a conversation with you about his awful behaviour that morning.
Weeks later, you confront him (in the dark while you were in bed) with numerous examples of his terrible behaviour.
He started behaving with more kindness towards you and the DCs. After a work event in March, he admits that the colleague was behaving strangely and wonders aloud to you, whether she is in love with him. When you tell him that you have been worrying terribly that he has had, or is having an affair, he makes you feel small again for even suggesting this. He stops a conversation about another's infidelity. He tells you the colleague has applied for a transfer and that he has tried to persuade her to stay. You ask him no questions about what has happened to the feelings he declared for (and to)this colleague last summer and he doesn't volunteer anything about that either.
He was behaving so strangely to your family at Christmas that they also suspected something was happening and when you tell them what has been happening, they agree that an affair seems highly probable.
Try - as others on this latest thread have with Estuardo - to reverse this. If you had come to this relationship with a history of infidelity, how would you have behaved in these circumstances? What expectations would your H have had of you if you had developed a friendship with someone that had resulted in declared feelings of mutual attraction?
I think it's your H's lack of empathy - and your fear of his reactions, that worries me more than anything in your situation.