Bio parents have a long time to set up their family environment. They develop together - your children learn to be children at the same rate that you learn to be a parent. It is quite possible that you too once went through a stage where every little thing mattered. But your child was just a baby, and gurgled and cooes his or her way through it and eventually you learnt how to let your child be themsleves without losing your own sense of self in the process.
Step parents don't have this developmental process. They come into the situation cold, and it is harder if they've not got any children of their own. You've learnt over the years what matteres and what doesn't, when your child is being a wind up and when they're just being themselves.
Some step parents just can't handle this. They take everything rather personally and tend to make an issue out of every little thing. From my experience on another step parenting board your relationship will go one of two ways.
Your partner may learn to relax, and take it easy around the kids, and to follow your lead on parenting. To get there, he needs to learn to take a step back, to disengage and not take things personally. He needs to feel appreciated for the step dad that he is, and not feel mortally offended that he's not being treated like a dad (especially if the dad is still on the scene).
Or your partner may just get worse, putting you and your children through a living hell before you finally kick him out but by then the emotional trauma inflicted on your children and yourself may not be repairable.
The only way I know of to go down route 1 is to be completely honest. Firstly with each other. He needs to tell you all the time how he is feeling, and he needs to feel comfortable that you're not going to judge him for it. A lot of what step parents feel is illogical, and they know it. It isn't anything that they'll ever act on for the most part or let affect the way they treat the kids but it is there. He needs to be able to tell you that he felt left out the other day because one of the kids handed you the tomato sauce first instead of him - he knows that is illogical and he isn't really upset about it, but he is still new to being a step dad and these are the sorts of illogical insecurities that all new parents go through. Just because the kids aren't newborns doesn't mean it is any different
Secondly you need to be completely honest with yourself. If you feel that the relationship is going down route number 2 then don't try and salvage it, or feel you have to try and work at it. But your kids welfare first and get out.