Hi everyone. I am utterly heartbroken and have no idea what to do.
Bit of background: I am 21 (will be 22 in a few days, happy birthday to me 
) and my fiancé is 23. We've been together for 18 months, engaged since December, and living together since March. I thought our relationship was perfect, and I love him dearly.
I sadly suffered a miscarriage six weeks ago which devastated both of us, but we emerged from it seemingly stronger than ever (at least I thought so).
Around four weeks ago (so around two weeks after my miscarriage) a woman, who my partner claimed was his cousin, sent me a screenshot on Facebook of what looked to be a sexually explicit message my partner had sent to her. However, she claimed that it looked like my partner's account was hacked and not to worry about it. Indeed, the writing didn't seem to be like his usual style and he also claimed that his account was hacked. Not having any reason to disbelieve him, I (perhaps naively) bought their story. He also proceeded to delete his Facebook account, so that gave me even more reason to believe him. He also gave me his phone to look at, and there was nothing suspicious on it.
Fast forward to today, and this same woman has messaged me to confess that his account wasn't actually hacked, they aren't actually cousins but exes, and that he has been sending her explicit messages and pictures for the whole of our relationship. She claims that she felt guilty after I had a miscarriage and didn't want to lie to me anymore. They haven't done anything physically and she hasn't sexted him back (she is in a new relationship).
She also says that he hasn't messaged her since he deleted his Facebook account and is genuinely sorry. I've sent him an angry text message (he's currently at work) demanding the truth and he's admitted to sexting her once, but denied it was going on throughout the relationship. He's begging me to forgive him and says he loves me, but I honestly don't know where to go from here.
Even his ex is saying he made a mistake, genuinely loves me, and should be given another chance, but I don't know wether to believe her when she says it was happening throughout the relationship or him when he says it only happened once.
This is my first serious relationship. I love him so much but am utterly heartbroken.
Sorry if this is rambling and doesn't make much sense, but I am so shocked and sad.