My fiance cheated on me

I am in desperate need of advice. I've been with my fiance for 4 years now and we've had a great relationship. Yesterday I found that a girl had sent him nudes. He says that she just sent it to him and he didn't ask for it but I feel that it's not true. He has basically begged me to forgive him but I honestly don't know what to do. I love him so much. I've been threw a ton of stuff in the last year and he has been the only person that I've had for support. He's literally the only person I have in my life. I'm scared that it happens again. I don't want to be with someone who cheats on me. Please help me make the right decision