Remember that your sobriety comes first. The last thing you need to be doing is thinking about this guy getting intimate with your girl constantly which is what it sounds like you're doing.
I'm not a shrink, but stating that you "can't get them out of my mind" is not the most healthy thing to be thinking about while in recovery. As hard as it might be, read the Big Book or other literature associated with recovery instead of focusing on what is going on in a back seat of car or whatever. I'm seperated and in all likelihood will be getting a divorce so I can relate to what you are thinking and saying.
Think about yourself first. Quit drinking for your sake not for other people's. Put your will into God's hands. If this relationship was meant to be, God will let you know at the right time. Maybe his plan for you is to meet a nice girl at an AA meeting who truly understands your situation. Time will only tell.
Keep the faith and keep the plug in the jug.