My Girlfriend Slept With My Cousin.. Pls Advice

So me and my girl have been together for about 2years, and she was the one who approached me. but regardless I love her and we are mentally married even planned a future together.
This morning she went to charge her phone somewhere, and I also went to charge mine too, so I just went to to her whstsapp and saw her complaining of pain due to the rough sex.
my heart has been shattered I dnt even know what to do... this my cousin is way older than me and is married, if I confront her Shey may tell him and he may try to hurt me. so pls guys tell me what to do?