Often times in life, something happens that makes us do things out of the ordinary and we feel out of place and do something stupid, despite our loved ones. It's a long shot, so I wouldnt recommend it, BUT maybe she is passing through a very hard time, felt like you two have lost the connection you once had, is feeling uneasy and needs a familiar face, hence the ex-bf. She may not have slept with him and may have said: I love him, but meant it as a brotherly kind of love....like the kind where you need a dad figure or a brother figure in your life type of thing.
She said she still loves you and she hasnt told her grandma to send over the divorce papers, so there is uncertainty and still an attaatchment of some sort.....Orrr she's playing you and stringing you along.
I wouldnt do anything hasty.
I would call her up and ask her to meet and talk things over. Lay it on the line. It's me or him. Do you have kids? how long you been together? is this worth it to you?