Hello totaleclipse, I hope you are ok.
I just wanted you to know that I have had a very similar experience with an ex and that you are not alone.
I was away on a course with work, a new job. Ex lived with me in my house. Everything there was mine. I came home for weekends from the course, but it was for 9 weeks, so he had much opportunity to do as he pleased.
I came back on a Friday night, the last day of the course and we went up to bed. I saw make up on my pillow, which was quite hard to see as it was a busy pattern, but I told him what I could see. He denied it of course, even swearing on his daughters life-despicable! I naturally then checked the sheets and found marks, which he still had nothing to say about, so I dropped the subject...
The following Monday night after I'd finished work(he was still at work), I was in bed watching a bit of TV when the phone rang. It was HER telling me that my boyfriend was seeing someone else. I really felt awful, but she was confirming my suspicions, so I held the mouth piece away from my mouth so she couldn't hear my breathing which was rapid and shakey...I asked her questions and she eventually gave up that it was her. She too was only a young girl...She described my bedroom, bathroom etc. I was mad as hell, but stayed calm to her...I managed to record a lot of the conversation and when he came home, I told him there was a message on the phone for him. He pressed play and I watched the colour drain from his face. He immediately said that he'd leave, but I got him to stay for a couple of months, mainly because if he'd gone then, I'd have been knackered financially, even though he paid very little toward the household, I'd just started a new job and was on emergency tax etc, so I knew I needed to have him there a while longer. I made him buy me new sheets for my bed.
We had a fantastic sex life, so he had no excuse for doing what he did.
When I did chuck him out, I literally told him that morning, gave him no notice...I didn't want him to be prepared in any way. He went. Stupid me though...I kept seeing him and I ended up pregnant. I don't regret my son, but I should've been able to break it off cleanly.