Wow, I can relate, and I'm sorry about what you are going through. You have the right to feel the way you do....even if you feel bad for "losing your cool." We all lose our cool sometimes, but in this case, many people would lose their cool if they were accused of something to some degree. Do you feel that you over-apologized? Sometimes I have a tendency to do this to not make waves, and as soon as I speak my mind to people who can be abrasive or even prone to aggression, I feel bad and worry about them abandoning me or saying negative things about me.

Please remember that in this situation, there is a need for her to take responsibility for her own actions. She should not have accused you of something unfair. If you feel it is something you can both let go and work through, that would help preserve the friendship. There is nothing wrong with confronting someone in an assertive way. In fact, it's healthy. That's different then continuously being aggressive and confrontational....which you are not, from what you are describing. Quite the opposite. If you're in therapy, maybe assertiveness is something you can work on?