While it is understandable that you would want to think the best of yourself in this situation, unfortunately it’s likely that the woman was correct in her suspicion that you had been two-timing her. The fact that the bra had been in your closet for months and belonged to a former fling, combined with the “once-a-week thing” that had become a “once-a-month thing”, suggests that you may have been seeing someone else and kept the bra as a reminder. Even if you had a good explanation for the presence of the bra, it is likely that the woman felt betrayed and that the trust between you was damaged beyond repair. It would be wrong to assume that the woman was wrong in her assumptions.