What a pair of utter idiots. It's so easy to show your best side and for everything to be all lovely when you don't live together and have never even spent nights together! As adults, how do they not know that? I never understand how people leave marriages for someone else, because you haven't been able to get to know them nearly enough to judge whether you're compatible or not. I think an exception can be where the marriage has been very abusive, but that's mostly men abusing their wives (I know not always, before anyone leaps on me) but I'm sure you weren't abusing your DH!
It's a very boring old tale. After a long relationship, your DH got his head turned by someone else. It's not rocket science or very original. And she's married! There really is no fool like an old fool, is there.
It's very very tough, OP - my husband left me too - but ultimately you really are better off without someone who can treat you like that. When mine bolted, I was so so devastated, and it IS devastating. All your plans for the rest of your life, your retirement, are thrown completely in the air and it's a massive adjustment.
But for all that - and I DO feel hard done by when I realise how much his actions have cost me, financially and emotionally - for all that, he did me a favour. I live alone now without having to pander to his moods and demands, and without having to spend my time with his relatives, who were very different from me....it's true that he has buggered my retirement, but I'm still far better off without him.
And I strongly suspect it will be the same for you.
Sending strength and hugs 💐