What can I do?
It happened to me, I just keep silence until my ex- apologized. I don't trust my ex-, but I still love my ex- at that time. So I can only pretend I let it go. I know my ex- still love/thinking about that person. But anyway, we separated and this was one of the reasons.
If you still love your bf/gf, trust him/her and let it go will be the best option.