So my ex girlfriend just told me she slept with someone else. We broke up 2 months ago.

I have a friend who attempts to sleep with everyone despite loving her partner, almost like a placeholder because of the distance. Sometimes she'll tell the partner other times she won't. As toxic as it is, somewhere underneath that she's trying to replicate the bond she has with them until they're together AND when they are together, she's madly fixated all over again.

The point being, feelings and actions don't always go hand in hand. Feelings are complex and we try and soothe ourselves in different ways, some that aren't ideal. It doesn't reflect you, it reflects her.