This is a horrid feeling. I am R in my second marriage, but I remember vividly the feelings when my ex husband of many moons ago first brought a girlfriend round to my house to collect my children.
It matters not the reasons, why's and wherefor's when this happens, it is simply the feeling that that part of our lives is well and truly gone, and that our ex has moved on while we are still taking time to repair our damaged ego's.
My ex husband was never unfaithful to me and the reasons for the marital breakdown were nothing to do with infidelity, but I felt the same way you do now, even though I divorced him because he was a total w****r!
It is the feeling that we are no longer worth anything if they can move on so quickly that makes us feel that way isn't it?
Well let me tell you, you ARE worth so much more than a shitty marriage sweetie, and this will pass, trust me. There is absolutely NOTHING wrong with what you are feeling and nothing wrong with YOU!
It's a glorious day. Go out and have a coffee somewhere nice in the sunshine and take the time to enjoy this lovely day. You deserve it!