After years and years and years of lurking, THIS is the thread that finally got me to sign up to Mumsnet so I can comment!
I'm sorry this is very long.
OP, your story is so familiar. In my late 20s, I came out of a very bad marriage, and ended up in another relationship almost straight away. He was a lot older than me, good job, interesting, intriguing, etc. I was charmed. Completely. And he had two small boys he was raising on his own, after his partner had died. I thought I knew what I was doing. I was happy at first.
Long story short, within 18 months, I knew that the relationship was wrong, but I did nothing, believing it would change, he would change, the kids would grow up, leave home, something would change. What happened was that I changed. I was no longer the independent young woman I had once been. He just charmed and gaslit and threatened and cried and blamed and lied until I didn't know what to believe anymore. He was a complete narcissist. In the entire time I was with him, he never once apologised for anything. Everything was my fault. My one saving grace was that I had my own job and my own home.
I ended up spending all of my late 20s and 30s waiting... helping to raise two very difficult children/teenagers, putting everyone else's needs and wants first, while my then-partner chipped away at every remnant of independence I had left, and I let him.
At age 39, after having wasted nearly all of my fertile years, thinking things would change, I was diagnosed with cancer. The treatment I had ruined any remote possibility of ever having my own child, and I felt bereft. Something in me clicked, and I said to myself 'if you have only 12 months to live, is this really how you want your last 12 months to be?'. And that was the final straw for me. And it still took me another 2 years to leave the relationship because I had no confidence left and no friends (all my friends were his). I couldn't even leave the house alone except to go to work.
OP, I knew after 18 months that things were not working, and yet I stayed for another 15 years! I had so much anger afterwards about the wasted time, but believe me, once your life is your own again, you won't care about "wasted time".
Decades later, I still have moments nearly every week when I remember what my life used to be like, and I am absolutely gleeful that I have my own life back again, that I am still alive to enjoy it!
I think you know that things are not right for you and your DS. And I think you know what you need to do to make your situation better. Don't worry about the time you have invested, think about how much more time you have left to make a happy life for yourself and your DS. I am thankful that you own your own home, so many women do not, and are stuck. Your DS will be happy if you are happy.
Remember what they always say: when he shows you who he is, believe it. I did see, and I did believe, but I denied it. For far too long. No relationship is worth giving up your whole self for. I very much believe that you will need to make whatever changes are necessary. No one else will do this for you. When you are ready you will make the right decision for yourself. I wish you strength and happiness. You are worth so much more than what you are getting right now.