He cheated

Hi all. I need to tell someone who doesnt know either of us before i go mad.. So In january by fiance exchanged numbers with another woman... friend of a friend.. since then adament they have just been chatting as friends... long ass story but fastforward to last night i found out he had slept with her on aprrox 4 occasions that i know about.. i love him and.we have been together11 years. He says there was nothing in it he thinks he was having some sort of midlife crisis. There are other things that point to this too though like new car. Change hair. Change clothes etc. All started before she came on the scene.. help what do i do