All's Fair in Love and War...Or is It?

Love does not begin and end the way we seem to think it does. Love is a battle, love is a war; love is a growing up.
James A. Baldwin

Why are relationships SO hard? Why can't the fact that you love someone be enough? I have been in this relationship now for almost 3 years, and it seems to be getting harder not easier. Mostly due to the fact that our children aren't willing to allow us to be happy. His kids refuse to accept me, and limit his relationship with them as a way to get back at him for bringing me into the picture. I don't think he will ever be happy without a robust, meaningful relationship with his kids, and they won't ever give that to him while I am in his life. I want to keep persevering, but I wonder if either of us has the stamina. I know that I have made a commitment and I intend to live by that commitment. But, I also feel that I am at war against an insurmountable adversary, and it's a battle that I am constantly losing. Funny, even the syntax I use tells all...at war, battle, adversary, losing.

Is it fair of me to ask him to hold out and wait until his kids come around...IF they do? Is it fair of his kids to continually hijack their relationship with their father because of their anger towards me? I feel that my children, although they are constantly rude, and bitter towards him, are able to handle this situation better. They have not chosen to limit their relationship with me because they are angry at him. (I don't know if that is because of their age, or because they don't have another parent bad-mouthing him ever chance they get.) There are no easy answers for sure, but I still think we have something that is worth fighting for.