How can I put his past behind me?

When I started going out with my partner a year ago he told me about some of his sexual exploits. Now I can't get them out of my mind.

I keep imagining him with other women, particularly his ex-girlfriend who lives locally. I haven't met her but keep thinking I might bump into her. He has broken off all contact with her and says there is nothing left between them.

I am 25 and due to move into his house soon. The problem is that his ex-girlfriend lived there before me. The fact that they shared the same bed as we do makes me feel sick. How can I put his past behind me?