The reason men cheat is because they can. They see an opportunity, they need validation and affection apparently.
Some men are just huge opportunists who will grab any skirt going past their grubby hands. And no thought given to any consequence, and forget any accountability.
Whatever you attempt to say, do, change about you, what you say, dont say, has zero to do with them cheating. Zero. Its not you. So attempting to understand the reasons why they take a side piece, it a waste of anyone's time.
And, the side bit did not make promises at the alter for you, your DH did, so going after her is simply an ammunition for your DH to use against you, whenever he feels you are putting foot wrong. And boy have you been wrong in his books, bet you could write a good old list of all the things you are doing wrong, according to your DH. Do not take the bate. Its not fact.
So, regarding the divorce, him not wanting it, that is due to the fact that it is HIS assets, HIS life, His desires, HIS affair etc. His comforts. The reason he wants to stay at home, stay married has zero to do with you, and everything to do with the comfort you offer.
List as followes of things you may be offering
Wash his clothes
Cook dinners
Make him lunch
scratch his back
sex
The familiarity of you
Him not having to admit failure in the marrige by having you stay, because if you stay that means he is not bad. So another free ticket to stray. Again, and again,
Why would you go to the one who did you so wrong for the right thing to do? He has shown you loud and clear what he feels towards you, which you totally dont deserve, so hold off saving this one sided marriage and finish that divorce. A liar is not going to suddenly stop, an abuser is not going to stop, he is doing both to you. And, stop catering for him.
You deserve a life full good things, so stop allowing someone who feels so little towards you ruin you. Just stop it. And well done on proceeding with the divorce.