Magda, I too am very sorry this is happening to you as it is obvious you've tried very hard to find ways to compromise and make your blended family work.
I don't think it is fair though to say that blended families can't work. I know many that do work incredibly well even better than non blended families, but there are so many circumstances that gets in the way, many out of everyone's control and no hard work, willingness to compromise, love and understanding can overcome.
What I've picked up from your trail is that your children and his might as well have come from another planet. Your children are well-balanced, well-adjusted, comfortable in their boots type of children. There are these children that separation of their parents don't hold them back. Of course it doesn't mean that they don't their issues, conflicts and uncertainties, but overall they are happy with who they are and evolve in their lives with directions they are happy to follow.
Your OH kids are those kids from séparation who end up lost in it, not clear where they stand, with years of having been fired with different messages, unsure as to who they are, where they belong, what they want to do. This of course will have come from a mixture of the personalities they were born with, but also more likely their upbringing and all that happened before you came in their lives. They are no less capable of making a goof future of their lives as your children, but they will do so in a much less linear route.
In my case, I've got two children who fit in both categories. In essence, they have experienced exactly the same upbringing, yet when looking deeper, they haven't for various small variations that all together built a different upbringing. Added to this is their very different personalities, DD very extroverted, DS introverted, her very confident by nature, him very weary, made a huge difference.
My DD is like your kids, my son more like your OH's. It was easy to be around DD, easy to bring her up and despite the stresses associated with her, she was a pleasure to love with. DS was full of conflicts, grumpiness, laziness, anger, lack of appreciation, neglecting of his hygiene etc... He was much harder to love and like.
Yet I've always known was that what he needed most was patience, love and continuing to believe in him whilst giving him a bit of a push to get him going. It's worked. He is now gradually getting out of that unpleasant, self absorbed stage. I am discovering a wheel grounded young man, one with an incredible emotional intelligence, who is naturally very observant with an ability to analyse situations with great maturity.
My kids were very close when younger but grew apart when teenagers and the reason is that my DD was exasperated by his lack of motivation and positive attitude to life, whilst he found being around her a constant reminder that he wasnt as good as her and that she was so much more likeable.
I think your SC felt this way around you and your DC and the harder you tried to intergrate them, the more the difference hit them and made them feel inadequate. Their dad understood that hence tried to smooth 5bings for them because it hurt him to see them feel this way, but it came out as him being soft and letting them get away with things that you wouldn't let your kids get away with.
It was always going to be mission impossible for you because of factors out of everyone's control. Your life were too different and the kids too old to recentre to find a common ground. It is very sad you couldn't hold it because in most likelihood, his kids would have grown up, build their own life finding contentment and they would have come around, but then again, some of these kids never get to that point and continue in life discontented and feeling that the world owes them.
It must be horrible to leave the person you love deeply because of disfunction with the kids but there comes a time when the pain of coping with the constant tension is more than that of being without them. I wish you good luck.