I feel like my soon to be husbands step kids are trying to tear us apart: Advice?

I am really struggling. The guy I am marrying was with his ex for 11 years, married for 8, and she had two kids from a previous relationship. Those kids see him as a father figure because he was around for so long. They are 16 and 18 years old, both female. I have tried so many times to build a relationship, but it’s not working due to their mom being someone that doesn’t approve of mine and his relationship. The girls will constantly belittle me, use him for their own benefit, and ever since he and their mom divorced, they have become VERY touchy-feely with him; other people also notice how inappropriate it is. He says that it’s nothing due to the fact they are his kids. I love this man more than anyone I have ever loved, but I feel like his girls are trying to tear us apart, and I don’t know what to do.

I feel like it’s the age. I was 13 when my mum got together with her ex and man I did everything in my power to split them up. He came into the relationship with my mum having 4 kids. And he stuck it out, treated us like his own. We all gave him shit until eventually we all got along with him so well. Mum & him ended up having another baby & then split up and we all still keep in contact with him cause he is like a father & such a good guy.
All I am trying to say it to stick it out, relationships get hard & 100% harder with previous kids, doesn’t mean you have to leave your fiancé because of other people. But it also won’t be a walk in the park, hopefully you guys just come out stronger. Good luck !

You need to consider your position in his life. If your place isn’t protected by him now, what are the chances that he will make sure your place in his life is protected later? None! This needs to come from him more than from you. Without his input, you will never be able to do anything about it and it will only get worse with the passage of time. Reconsider your choices.