Betrayed

Two days ago, I found out my fiance HAD been texting an ex co-worker (female) some inappropriate messages. They started texting in late January and stopped texting in late May. Everything from "I miss you" to "Happy Valentine's Day." And wanting to crawling in her bed and holding her. I found these texts while he was in the hospital having surgery done. He has said he is sorry but can't tell me why he did it. I'm so hurt, I can't look at him, touch him, etc. I don't trust him, I don't know if it was every physical between them. I need some advice, please. I can't forgive him right now.