hold, I'm thrilled you seem to be starting to get it. Do you see how weak it is to claim to be working on his marriage and putting you first, but then also contacting her and pining for her and seeing her at parties and lying to you about it? Do you see how sniveling and selfish and 'anything for an easy life' and small that makes him?
-" I would frequently pick him up on this behaviour before but now I feel too insecure."--
This comment worries me greatly. Can you see that you are actually desiring the OPPOSITE of how a marriage survives an affair. It is VERY VERY difficult for any marriage to get over a romantic affair. In my experience, the majority don't (as opposed to a purely sexual affair or a one night stand or an emotional affair, for example). And it is for exactly these reasons. But when they do, it is when the cheater takes MORE responsibility and criticism of their character as a result of their actions NEVER when the cheated upon changes their behavior or feels judged.
The very fact that you think he would go to the OW speaks volumes about your fears and behavior, and frankly, their relationship. You see like the one consumed with fear of being left, whereas it is HE that should feel that way. An I am certain he doesn't. Whatever lip service he pays to this, we can tell here on an anonymous message board that you have absolutely no intention of leaving this man, so he who knows you well must feel that completely.
It also concerns me that you are starting to 'get it' only once I mention detaching as a strategy to 'keep him'. It is almost as if you can only bear thinking about separating as a technique to keep your man, which is sad. I get the impression there is something about you that would find the idea of being alone shaming in some way, either in regards to failure, loneliness, imperfection or weakness. I would look at this and how it is shaping your decisions.
Do you really want to be with a man who admits to you that he is mourning the OW, but who you believe would not mourn you? WHY? Is it, as I suspect, because you know he would never mourn you because you are safe, reliable, ever there. He never fears losing you. It is the OW, he has lost and therefore has real feeling for. There is a good book called The Passion Paradox (sometimes called The Passion Trap) which explains why we feel more in love with people who are rejecting us. It is really worth getting and perfectly outlines power issues in relationships. At the moment your DH has all the power and no fear of loss. This situation is very very bad and it will never recover unless YOU act.