Facebook message - trust

My partner and I have been together for over 4 years, it's been rocky to say the least with him spending a lot of time in Spain with his mum. I finally thought we were doing well he's back in the UK permanently for now he's finally working and helping with out 1 1/2 year old daughter. I've never had any trust issues whatsoever never but the other day I received an email with some messages sent beginning of October whilst he was in Spain. They were to an ex talking about a time they'd had sex before he met me. He was a bit graphic and said she loved it. I've seen the conversation and there was nothing about now and nothing like he wanted to see her again etc it was just about the past. I'm absolutely gutted, we talked and he has said he's never cheated and I do believe him. I have decided to let it go but I'm having trouble getting the thought out of my mind. We haven't had sex in weeks he's put on weight and says he doesn't feel the urge and that it's not me. We've also just found out I'm pregnant again despite being on the injection. He tells me he loves me and wants us to be together and work. I love him so much how do I move on I know time is the key but am I just papering over the cracks.