I always wondered that too,, my H's OW is best friends with my SIL.... double . He spent 2 months in Mexico this spring and I made it thru he also visited in October and December for three weeks...

I felt like I was going to pass out when he went in October,,, I made it thru. Then in December somehow I did mention her.
he said she hasnt been here dont worry..
and I said this...

" I MADE A DECISION WITHIN MYSELF NOT TO WORRY ABOUT HER AND WORRY ABOUT YOU. YOU NEED TO KNOW TO DO WHAT IS RIGHT AND I CANNOT ALWAYS BE WITH YOU. WHEN YOU ARE NOT HERE I CARRY YOU WITH ME WHEREVER I GO AND ACT ACCORDINGLY AND I EXPECT YOU WILL DO THE SAME FOR ME.IF I CANT TRUST YOU THEN WHAT DO WE HAVE? AND IF YOU WOULD EVER LET ME DOWN THEN I WOULD HAVE TO MAKE A DECISION. GO FROM THERE YOU KNOW ,I CANT DO THAT EVER AGAIN. IT HURTS FAR TOO MUCH."

H: " I KNOW." ( IN A VERY QUIET SOFT VOICE)
In the spring when he was there for two months I thought if he sees her he will know that she is NOTHING COMPARED TO ME! I FOCUSED ON THAT. Or I would have been useless in my daily life it would have consumed me. It was hard but when he saw and felt the trust I gave him it helped him see how strong I really am. he told me once the more you trust me the more I want to be a better man for you....
I too wish there was some sort of delete button for :OW IN THE BRAIN.
I know it is easier said than done but try to not let her invade your thoughts too much and be the unforgettable Woman that you are. He is with you and HE chose you... be the Woman he loves and be strong.... one day at a time *( HARD AS HELL I KNOW BUT NOTHING IS IMPOSSIBLE ) I KNOW ~You can do it!
P.S.
MAYBE YOU ARE LIKE ME WHEN THINGS SEEM TO GOOD TO BE TRUE YOU THINK WHEN IS HE GONNA DROP ANOTHER DOOZY ON ME?
Take care sweetie.
GOD BLESS...