Oh gosh LMHF I can relate to so much of what you say!
I was going to ask, are you sure you actually have issues? I thought for a long time that I must have something very wrong with me, that I must have done or not done something to end up in an abusive relationship but now, thirteen months down the line, I don't think I have any issues at all. The only issues that I did have were the ones he caused me to have! Now he has gone they are gone too! I would say the only thing you should be focussing on is being happy! You are free!
Don't feel obliged to put yourself through the mill looking for issues to work on, you had enough of that while you were with him.
You are not on your own in feeling idiotic for having put up with it for so long. I was with mine for 11 years and if I knew then what I know now I would spot the red flags fro day one! Back then though I had no reason to doubt anything because I had no knowledge or experience of such an abusive relationship. In the early days I mistook his control, jealousy and possessiveness for love and because it came in cycles it was so easy to get sucked in. I liken it to the frog analogy, if you put a frog in boiling water in will jump straight out but if you put a frog in cold water and gradually heat it up it will stay in there and wait it out. So, don't you go feeling stupid, you were heated up so gradually you didn't didn't feel the need to bail until the heat got unbearable. Then you did the sensible thing and got out so yes, you should be applauding yourself 
As for you feeling lost, I felt exactly the same way but I think mine was due to being so excessively controlled, he totally dictated my life so when it came to living my life myself it was difficult. Also, I was now just 'me' and not a part of something, plus all the drama had gone out of my life, it was quiet and peaceful but it felt strange. I know this sounds crazy but I think I felt lost without the abuse, normal, everyday life was so alien to me.
I've seen you give fantastic advice on MN but I do know what you mean about feeling like you don't have a place here any more. I used to be a regular poster but took a step back because I felt the same, hence my namechange. But you do have a place here LMHF because you have valuable experience that you can share. You might find though that further down the line you just don't want to talk about it, you don't want to bring it all back and that's a good sign I think.
I can totally relate to how you feel about all the years trying to negotiate with him and the realisation that you didn't know him. Once I had left my ex the enormity of what I had been living with hit me and it was far worse than I had actually realised. I had minimalised so much of it in order to cope. I went into shock after I left, I truly could not believe that I had been through all of that and survived. But you are right, you probably didn't know him at all. My ex is like a complete stranger to me now, I can't believe that I spent 11 years with the man.
The needless destruction of things, yes, I can relate to that too. My ex would destroy anything I got pleasure from. I used to love my garden, growing seeds and plants in my little plastic greenhouse. Until one Sunday morning I saw everything loaded on the back of the truck, he took it all to the tip. Just like that, without a word to me. The man wanted me to have no life at all, he took away everything that gave me pleasure or happiness.
I feel like you too in that I am glad it is over, I am pleased not to be with him any more but at the same time I have a sadness that it didn't work out. I know it's an irrational sadness because it could never have worked out with him because he wasn't a partner, he was a dictator, he wasn't a nice man, he wasn't pleasant, loving or affectionate, he was a tyrant so even I have trouble understanding why I feel this sadness. It does get better though, I am a lot less sad now than I was a month ago and in another month I expect to feel even better.
That is wonderful news that your DS is showing signs of being less stressed. He will go from strength to strength I am certain of that. My DD has blossomed! When we left she was a very withdrawn little girl who lacked confidence and never smiled. To be honest with you I thought she had inherited some of his traits but when we were in Refuge she just came out of herself, within weeks of us leaving she was like a different child, she was laughing and playing and having a whale of a time. The child support worker explained that although I had been doing my best to protect her DD had picked up on everything, that although I hadn't realised it I was living in a depression and so DD had taken over the role of carer, she had been looking out for me but when we moved to refuge and she could see that I was happy and relaxed she could be herself again, she could be a seven year old little girl. That was the best thing about leaving and gave me the strength to stay strong and not fall for his charm and promises when he was trying to get me back.
I'm on the health kick too, I've got 2 stone to loose! Once I was free of him I discovered the joy of food and so over indulged a little 
You know LMHF, you have all these things going on in your head, all these questions and possibly a few doubts but you really do have the answers too, you are doing just great you really are, I think all you need is a bit of validation that you are heading in the right direction and I can give you that, you are sooo heading for happiness! So, give yourself a break, don't be so hard on yourself 