Have name changed for this, but I am here a lot!
Have been married 20+ years. For the first 15 or so, sex wasn't that frequent, but was very very good when we did get round to it. For a long time, we always had good sex when I initiated it, but could go weeks without if I didn't,as DH never would.
We talked about the reasons for this and it was basically that DH was scared of rejection, which was a fair point as there were times when I just wasn't up for it, whereas DH has never rejected me. For a while that was OK, but it started to feel like DH didn't want me. He'd play if I asked, but he didn't seem to "want" it iyswim
Then I got close to a colleague 
and couldn't be bothered with DH anymore. It just seemed too much like hard work,when my friend made it clear how attractive he found me. Nothing ever happened and I've been avoiding him for months, but whenever I do try to get intimate with DH, OM keeps popping into my head, which makes sex with DH seem all wrong.
I love DH, we are incredibly close friends, there is nothing he doesn't know about me - we even talked about my colleague. I love spending time with him and curling up on the sofa with him, we laugh a lot together and are never short of something to say to each other, but I'm really struggling to find him attractive / not to be distracted during sex. What can I do to sort my head out?