Maybe try explaining to your DH that his son is 17. He is growing up and is his own person. Your feelings for your husband are separate from your feelings for his son. You see his son as almost a grown man, not as an extension of your husband. Some of these parents see the kids as extensions of themselves, so if you don't like something about the kid, you are insulting the parent. But that isn't true and the closer to adulthood the kid, the less true it becomes. Your husband has to let go of seeing and treating SS as his "baby." Maybe if you frame it as a good thing that he is growing up and a normal thing to treat him as more of an adult, your husband might get it. 

It won't take one discussion.  I had a very similar convo with my DH about 4-5 years ago. I was distanced from everyone when the SDs were with us. Because I had zero say in anything that happened or could happen. I was not considered whatsoever. So why should I play "happy family?" Because that's what DH wanted, still does. It took many hard, long conversations with him. I stayed away from accusations. Used real examples from things that happened. Talked about how I felt and how I saw myself. When he got defensive, I remained calm, let him talk it out. Then either tried again, or just told him that the conversation needed to end for now because he was being defensive and not really listening or trying to understand.

This was around the time I "noped" out of it all. I suspect he spoke with his counselor about it and did some reading on his own - he does thank me for what I put up with and still do. And I never ever kept my mouth shut when it was a serious thing that effected me directly. He had to deal with it because I put it in front of him to do so. It all did have ramifications on our relationship. I think we truly lost some time together because he was so worried about the SDs that our own needs were put aside - I saw that happening but didn't then have the words for it. We have talked about that now and he has regrets and sees it now and feels bad for me. He does work on it and that's what saves it.

If your SO categorically refuses to have a rational conversation with you about SS's behavior and how it effects you, your daughter AND the entire houshold, then you for sure need to think about how to have other potenially difficult conversations in the future regarding SS. He's setting the stage now that you come 2nd in everything and likely it won't end.

Sorry but I think your husband is being emotionally manipulative. 

"Am I too far gone." 

I think this is more on your husband and his inability to be an actual parent and effect change that positively helps the entire household. 

He's a lazy ass trying to make you feel bad enough to suck it up and continue "just dealing with" all of the stuff you have repeatedly told him needs correction. 

This is not a you problem - it is a husband problem. 

Make you act the old way.  Just asept it as you did in the old way.   NO. Draw the line in the sand. And it's your way.  Not the kids way.

DH has to realize he can't make everyone happy. He going to upset someone,  it's you, or his kids.  If it's you then he can move in with his kids.  Have happy life happy kids. ???

I honestly can't answer your question. Are you too far gone? Only you know where that exact line is and you'll know w/out doubt when you know. But right now you are in a 4.5 person household with only 3.5 participating in keeping it running as a family. Your husband knows this, he knows it but will not step up and ride his son's tushy to be a family member.  Sadly what brought you to this point is not a you issues, it's a them issue. Them being him and the son he refuses to deal with. Does he offer a solution beyond you just sucking it up? How does your daughter feel about all of this? Kids are sensative to emotions and tension. Where do you see your household in five years?

One thought I do have, after going back and reading your first post, is that your partner deals with his son by proclaiming himself too sensitive by his words and actions. It's his cop out to avoid dealing with the real problem going on.. his son

I suggest you, after the baby arrives, revisit my questions. I wish I could offer you better words but please know I read what you post and my heart hurts for you, your kiddo, and your soon to arrive kiddo. 

Be direct, be confident, and do not let DH gaslight you with this "I don't like that you are pulling away."  Point out that his failed parenting is pushing you away. You are not pulling away.  Two very different things

Someone who has failed as severly to be your mate as he has, is not someone you can be attracted to or continue to make a life with. He needs this clarity and he needs his idiot nose rubbed in that stanky spot in his life's carpet and you need to move on with your life adventure with he and his failed parenting experiment in your past.

Stop punishing yourself for his failures. Put the failures where they belong.

Take care of you.