My tuppence worth...
As a poster said to me on one of my threads - 'right, shields up!'.
Do not engage with him. You're too fragile and may understandably say things that don't reflect the dignified and lovely person you seem to be from your threads. Plus - it will drive him MAD!.
As long as you engage and show your distress he is getting what he wants. Proof that he is desperately important. He already thinks that, he's got his new 'girlfriend' so he thinks he's hot, irresistible, he caught her when everyone else wanted her blah blah blah and you left broken and devastated because he's so damn fantastic.
Cut off his supply of attention from you. Have ice in your veins when dealing with him. He does not care about you let alone love you and that is SO painful but don't give anything away.
You'll make your own decisions but I wouldn't contact HR or go out of your way to get the truth out there. It WILL all come out in good time. If people ask, keep it factual and not emotional - imagine it's a Police statement for example if that helps. So 'I was shocked to find out of his affair and see evidence of it in pictures and texts. We were planning the future on our recent holiday so I'm surprised to hear he felt unhappy'. Don't give too many details or add the emotional impact.
You have every reason to feel angry and bitter but don't act from those emotions - he WILL use it against you. I have seen friends go through this and put stuff on FB or tell lots of people the sordid details and it resulted in the H rewriting history EVEN further and say 'see what I've been dealing with for so long, trying to live with this horrible angry person'. Or 'I left because I was unhappy but she can't handle that so she's having to make out I was having an affair and OW was only a friend before we split but was there for me and we got close'.
The people that matter to you will know the truth. The others don't matter. They really don't.
Keep doing what you're doing. You are dignified and strong. He will not be able to look back at his behaviour and be proud - you will.
And he will want you back at some point. Even if he wants the OW he will try it on with you at some point because if you seem to be handling it all well, he'll want to prove to hid ego that he could get you back if he wanted.
Remember all that you're going through now and his callous behaviour and cut him off emotionally from you right now. Never let him get a foot in the door.