It's wrong but why can't I stop?

So i’ve done a search for cheating/emotional infidelity, etc. And if it is such a thing then YES I’m guilty of it. I want to know why it’s so damn hard for me to just break all ties.

Here’s a litle background info. We are co-workers, both married and spent a lot of hours of the workday together. she has since moved to another position but we remain in contact daily regardless of time of day. we meet at least once a week for lunch. she doesn’t think that we are doing anything wrong yet she has said that she wouldn’t be comfortable with her husband doing what I’m doing.

I know it’s wrong but I can’t get myself to cut her off and i don’t know why.