My advice would be
at this point
to stop treating you daughter like she is an adult
and by that I mean
she needs to not be involved in her mothers love life by your hand or hers

she doesn't need to continue to know this and what she does know should be dealt with with her counselor

teenagers emotions are huge and right in y our face
hence Romeo and Juliet
adults have more self control (we hope) and can be more rational...we have a better vocabulary and can express ourselves differently

My ex was vile and wicked
I told my kids he was sick
sick in a way medicine wouldn't help

they are older now and they still don't know the nitty gritty
nor do they need to

and yes
you don't have to tell the whole truth when your daughter asks you something

why are you home early can be because some stuff came up so you decided to come home (the truth)
change the topic
if she presses it is ok to answer that it is something that is between mom and dad and she doesn't need to know about it...it is adult stuff

and Jack...thank you for clearing up what you meant
I beleive we all would walk through fire for our children and sometimes that means (usually that means) we have to humble ourselves to do what is best for all
if we truly did that more, I believe more healthy adults would develop