I wonder if what is so hard to let go of is the "idea" of him. You were together for 20 years...that's a long time. I have a friend who said they don't miss their ex, they miss what their ex represented - the partnership, the companionship, the family unit, etc. I'm guessing this might be some of it and even if it's better, it's still a loss.
When there's another person (in our case, woman) involved, there's feelings of rejection, too, even if you were the one that ended it. I think your feelings of being used are natural, too. That's something I struggle with. The reality is it's not about us. It isn't because there was something wrong with us...it's because there's something wrong with them. Normal people don't behave the way they did. Unfortunately reality and emotion aren't always on the same page and that's where the difficulty lies.
As for your son meeting her, there's nothing you can do about that other than to try and make it as comfortable as possible for him. As long as she treats him well, that's all that matters.
I'm sorry you're hurting. It's only been 5 months...give yourself time, let yourself hurt and grieve but keep moving forward while doing that.

I agree with melissa87...I think I've now realized that I miss the idea. I'm able to look at my stbx much differently now (we were together 20 years) and finding that I miss her much less, but I still miss the idea or expectation that I had about marriage and family. Also the "it's not about us" thing is true but very hard to get to and wrap one's brain around. Instinctive emotion is there is something wrong with me, why doesn't she want me, what can I do to make her want me. That is a strong instinct but sooner I was able to accept everything, the sooner I could grasp that "it's not about me" concept. Our therapist told me that I would be surprised how much this is not about me...and she was right. Self esteem took a huge shot and dealing with the feelings of rejection but after almost 9 months now it's gotten better.

Thank you Melissa and whatthe321. I think both your comments on "its not about me" hit home. I did logically know that there is something wrong with him, but nevertheless I do fall into the trap of "what was wrong with me that he couldn't have...." Especially when I see him making changes that I longed for during our relationship. I'm waiting for the first stirrings of acceptance to come along, but I think I'm still stuck in grieving and a bit of depression right now. Thank you both, you're thoughts have really helped today.

@melissa87 and @whatthe321, you have both captured my thoughts and feelings exactly. I took pride in the role of husband, father, provider, lover, etc. Though I have some visitation with my son, it is the feelings of loss of my identity around these roles that I let define me, and my sense of self-worth, that have hit me the hardest.
I'm still raw from "round 2" of being left by my ex, only a few weeks into the 2nd rejection after a 12 year marriage, so emotionally the sense of loss is overwhelming. I realize that manifests itself in wanting my lover back, and my roles back. Emotionally it is excruciating. Intellectually, I realize that I was not solely at fault regardless of her criticisms or blaming me, in fact, little was really about me, and that I am better off without her as I deserve to be treated much better and have a supportive partner. Still, that does little at first to ease the pain, loneliness, sense of rejection and loss.
I also find myself at night thinking of her and her new man, and of my son being introduced to this exciting new man that lives in the Florida Keys and runs an exciting sport fishing charter business. I know my son will love going out on his boat. I have to get to a place to be supportive of my son without jealousy and resentment of the new man in my ex's life.
All of these changes and adjustments will be difficult to do for all of us, but we will find the courage and strength to be the better person and let go without resentment. As they say, "detach, with love."

I hate that we are all going through this, biut it is a comfort to know that I'm not the only one with these feelings and that I am not being an obsessed jilted woman.
mlewisct, the jelaousy and resentment plays a big role with me too. How to detach with love is a work in progress - right now I am just praying for the deisre to want to pray for them and wish good things, and I hate that as I am normally a very kind and forgiving person.

@tcl18 - having said that, acceptance wasn't easy for me. I spent the first 6 months trying to fix me to what she was telling me. I expected that when she saw my changes then of course everything would be right again...she doesn't really want to mess up the family...right? What I learned later on is the ones in our situation that get blamed for so many things (and left me scratching my head) frequently turns out to be a guilt trip for the other's transgressions. When I was just finally mentally and emotionally exhausted, I gave up. That's when I started getting angry which then helped me to gain acceptance. I never physically acted on the anger, but rather using that energy internally. I'm not an angry person so it was foreign to me, but focusing on my anger really helped knock down those sad and self esteem feelings and let me do what I needed to do to get me good again. I'm still a work in progress...but I do see progress.

@whatthe321. Exactly right. While we scrambled to "fix" ourselves and try to become prefect to save our marriages and not lose our spouses, in the end it was really futile - her making me dance so to speak. At least in my case, I now see I could never meet all the changing criteria, and that while there is some truth mixed in and some things I need to work on, most of it was her rationalization journey for leaving. Each new rationalization was another way I was failing, and I internalized all that (mostly BS).

Like you, I now am starting to see it for what it was and it does make me angry - for all that time, effort and torture, for her discarding me, etc. I am also not an angry person, and rarely even raised my voice, so this anger is a quiet anger that just acts as a beacon to help me get my head straight. I don't wish her ill in any way, and will always love her, but I can now see that the torture I went through clearly shows her mind was made up, and I will be better off without her, regardless of the current excruciating pain.

Yes, "scrambled to "fix" ourselves " - while I asked him to leave, in hind sight I have been doing this - if only I'd more supportive, more understanding, more patient, more flexible - playing that mind game of blaming myself. But I know that I was the one who kept trying to work things out, asking him what I can do to help him make things better, trying counselling etc. - it was him who choose to ignore, blame, etc. and ultimately fool around. I do have to acknowledge my flaws and how they may have contributed to problems in our relationship, but ultimately I was all in and he wasn't.