OP - I think you are wanting advice on how to not feel sad about losing something you wanted and have denied yourself (an emotional and sexual relationship with another man). And the reason no one has given you a great deal to go with is because you seem to view the behaviour you have engaged in as ‘a mistake’ - as if it was an accident? Or that you stopped before making it much much worse so you’re entitled to feel good.
Your feelings are still about you and not having something you wanted, and feeling sorry for yourself. What you haven’t really expressed is remorse: for the deceit, for the lies, for the betrayal and disrespect and for the callousness which you will have directed to your DH.
The hurt he would feel is agony compared to the rather teenage rejection drama you are experiencing for someone you’ve known for a couple of months and who is, it seems, even more callous than you.
So if you really want to gain some emotional perspective - tell your DH how you have lied and deceived him, and why, and how you were driven by your desires, and that they were stronger than your respect for your marriage. And how you backed out at the last moment and feel sad. And when you experience the utter devastation of the emotional upheaval, the pain and damage you have done to someone you say you love, then you may understand how selfish you have been to continue to think of your thwarted desires as worthy of sympathy. Because it is the pain of betrayal that many other people have experienced or witnessed on these pages.
If you live through the pain of a continued marriage that will never be the same, or a separation and divorce triggered by a selfish act, then the depth of that emotional experience will reveal how shallow and fleeting this current sadness for a dickhead you hardly know actually is.
Or - you could just say nothing and suck it up and get away with it? And take something from the fact it went no further, and become someone better?