Btw I can see two separate things going on here.
Firstly, there is the jealousy you feel towards your DH and his sons close relationship, which I can understand to a point. We all want to feel as though we are the most important person in our DH life. But, you would be wrong to try and break up their relationship regardless as to how you feel. I have realised by my own SiL's behaviours, you can be a rubbish husband but a wonderful father and we just have to 'suck it up' for the sake of our grandchildren.
The second thing seems to be, that you are putting your hatred for your own ex onto your SS. Because your ex didn't have anything to do with your own children, could that be a cause of your dislike of your SS, because he has chosen to keep his children with him? I may be wide off the mark here, but that's what jumped out at me.
I do think some sort of counselling to explore your feelings for your ex, your SS and your husband in case they have all became entangled in your mind. As someone who also suffers from mental health, I know how easy our thoughts can become confused turning something innocent into a massive burden.
Good luck and hope you can get help to help you see things better. And agree with others, it sounds like your DH is a good one, don't do anything rash re leaving him until you are in a better place xx