Did she ever date other guys or mostly just you? I have seen cases where the couple started dating each other young, get M, and then years later decide that they might have missed "something" by not dating more people.
There has to be more to this. Even if it is MLC, I think you would have surely seen some sign. Like I said before, a woman doesn't get up one day and decide she wants to leave a M unless there is something out there that is pulling at her.
Was the GF around at the time you noticed that kiss without any feelings?
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!