nataliemej I do sympathise with you. Being a step parent is hard and pretty thankless. Try not to hate though, it poisons you.
I have a DS and DD and my DP has a DS from his first marriage, all teenagers. We have been together 7 years. It is very hard. My kids get on well with and like my DP, he works hard to keep the relationship.
It is completely different with my DSS, and has deteriorated over the years. When he is with us he does not come out of his room, his DF takes his food to him there, he will not talk to my kids (who he has known his whole life and used to be close to) and will not come into a room if I am in it. We have not spoken in months.
My DP totally understands how and why this has come about. We make time for each other, if the kids are with us we will still pop out for a curry on our own and bring them back a takeaway, go to the cinema too. Teenagers can look after themselves for an hour or so. It teaches them responsibility and trust.
One thing we do which does give us back some me time, is make sure we have a weekend away together two or three times a year. It has made a huge difference with being able to cope with the stress at home as a couple.
Maybe suggest that you have a weekend away together once a year. Surely that can not be too much to ask. What do you do about holidays? maybe you can have a week away even, would that work?
Also you should try with your DSD, how about taking her Xmas shopping to get something for her father? Small steps do make a difference.