Why do I resent my stepson?

He is nearly 16 and has lived with me and his dad for 5 years. Used to get on great with me and my three kids (9, 13 and 18-we all live together), but now I almost feel hate for him. His dad would do anything for him and he has always been spoilt but not had any social upbringing to speak of from mum(she had the party girl lifestyle and left my partner and his son(then 11) for a younger man, we met soon after). He has no manners. no respect for me or his father(or anyone except strangely his mum). Access has always been difficult, my children have arrangements to see their father that we all stick to so everyone knows what the plans are. SS does whatever he likes and dissappears as soon as mum decides she wants to spend time with him. SS doesn't tell us, he just goes for 3 or 4 days at a time, often missing school. To me this is unnacceptable, their are other children here and we need a family routine. He is backward in social enviroments, untidy (leaves the toilet in a disgusting mess which I hate-despite being constantly reminded). He is very materialistic and demanding and very immature. Mum calls all the shots, but she refuses to help financially or stick to any arrangments, If I bring up these issues with partner, he says shes his mum, he wants his son to be happy and that I'm just nagging. He sayswhat do you expect me to do and that his son will always come first. I resent SS and feel sick when I come home and he's here. I have brought up three of my own children with boundaries and guidelines and I'm proud of them, they have been thru a messy divorce but are settled, happy and well adjusted. Partner dismisses probs, cleans up after him, rearranges plans and gives in to him all the time. How can I be more forgiving, I've tried everything but feel like I'm up against the pair of them (plus the mum)! Any constructive advice welcome, being a mum is the hardest job, I've done it for 18 yrs and just need some help, these negative feelings toward him are making me miserable!