If you love the person you’re married to and you want to save your marriage you stop thinking of the children as being “not your children”. Biology alone does not make someone a parent and all too often biological parents themselves are frustrated and fed up and sick to death of their children and can’t wait for their children to grow up and get out. Children present challenges even in marriages where both partners are the biological parents to the children.
When you have a sense in your head and heart that the children aren’t yours so you don’t have to care about them, you don’t have to take any responsibility for them, you don’t have to like them, try to understand them, try to be a parent to them, you do more damage to your relationship than any damage you think the children are doing.
If you love your spouse you and you want to save your marriage become willing to love your spouse’s children as if they’re your own. Being willing to be a parent to them in every sense of the word is the first step. Will it be difficult beyond that point to build up love and genuine interest and caring for your spouse’s children? More than likely and especially if the children hate you; but remember they could be your biological children and still hate you. You wouldn’t get to contemplate leaving because you don’t like your relationship with them. It would more likely be your reaction to the problems you perceive the children to be causing that will cause a marriage breakup than it would be the children themselves. They came as part of the package. You accepted all the responsibilities of a step-parent when you married their mother or father.