My boyfriend of 4 years cheated on me, PLEASE HELP!! :(?

My boyfriend forgot his phone in my car and while i was going back to his house to take it, i went through his messages and a saw a conversation with a phone number not saved in his contacts so I opened it up and come to find out that it was a msj with an adress and he responded saying im outside. As soon as I read it i was soo madd and hurt I drove up to the curve of his house and tol him that we were over and left super fast. Then i was crying and crying and hurt and back stabbed then I thought to myself " wait maybe its a friend or something" I googled the adress and it was a Hilton Hotel!!! :,( ohhh jeezzz ma head started spinning and my heart started racing. I couldn't belive it!! :( , Then like 30 mins later he text me saying that im probibly imagining things and that I go through his stuff without reason, and I didnt text back then 20 mins later text me again asking me if I was going to respond, and i still didnt text back. Im sooo hurt I love him soo much but weather or not he had or not, he drove over to the hotel!!! It hurts me soo bad beacuse we had plans of hopefully one day sharing a home with a family and everything. We were like a married couple. We do everything togther his 21 and im 19. I havent called him or text back, this just happend last night, but what do I do my heart dosent want me too leave him and my head is telling me he messed up and thats the end. This has never happened in our relasionship. Please any advice would be great!!! I just don't know im very hurt. ;( I cried my self to sleep thinking how could have this happen we are a very serious couple non of tge immiture drama. Also a part of me wants to hear his side but fo what he went to the hotel and responded to her saying im outside. Please help me :(