I think it's a little harder to let go when you have children (especially young children) in common. You still have a connection, are likely to encounter each other at significant life events, may still have to communicate and hammer out agreements (or hash out disagreements).
I haven't needed to entirely let go of my first husband, and have no desire to--I still consider him a good friend. OTOH, when he has been going through some emotional turmoil lately, I was able to listen and offer support, but not carry around the burden of his feelings--I was able to let him work them through in his own way. So in his case, I think I have let go in a healthy way.
With my second husband and with the last guy I lived with, I feel like they are past history. I don't feel a tremendous amount of anger or regret, my blood pressure doesn't go up when I think of them, I don't miss them. I had some nice times with both of them, and some truly awful times. Some of it was them, and some of it was me. I feel a lot of acceptance, which is a form of letting go.