Speaking as someone who had been together for a very long time with my husband, I too thought this could never happen.
Although he was capable of being hurtful I never thought I could be hit with 'that' kind of hurt.
Sometimes its a combination of different factors, for us it was buisness differences, illness on both our parts, and too much responsibility for me to mop up financial messes which led to resentment, to name a few.
The marriage was in a bad place, but nothing I thought we couldn't work through as we had done before, throughout the years, but not this time.
I often wonder if he did it for spite or if he was just so encaptivated by her that everything else didn't matter.
It didn't last but neither did my love and respect for him.
All I can say is if you do honestly believe that your partner 100% would not hurt you this way, I hope you are right and will be forever more.
It's a pain like no other and takes an inordinate ammount of strength to overcome whether you stay together or part.
No one ever expects it to happen, whatever age you are or how long you have been together, the first time kills.