No one can tell you what to do, but that being said I would ask you to consider what is there in this relationship for you? Are you her brother? What kind of person goes off with someone else and wants to stay married. Sometimes we do love people and are no longer "in love", caring about someone is not the same as being "in love".
Marriage is not easy but taking off with someone else says to me your relationship as a couple is over. I know nothing of your situation but it sounds to me like it is time to move on. It is nice if you can do so without ranker.
I am sixty two years old and I have seen many relationships and had a few myself. Sometimes when it is over it is hard to say so -- maybe that is her problem. Even so, is it fair for you to sit and wait to see if see comes back, and if she has not gone wait until this fling is over?
Again it looks to me like it is over, but it is your life, your relationship, and your decision. What could you possibly build from this situation in the future -- only you can answer this question, what do you want to do?