I never expected to be Dad from day 1. I love those kids however, and my heart aches for that first time I get called 'Dad'.
But to my kids I'm 'Shauny'. Their mum and I were together for 2 years before we got married, 100% custody during, but when Bio-Dad was sober he would get day visits. He's the manipulative kind. If he put in $15 for a $200 birthday present, the moment he was alone with the kids, he would tell them he paid for it all, even though hes an alcoholic that refused to pay for child support, let alone a present.. And then they'd be shy around me because he told them I'm a X Y X and if they were alone with me I'd do Z. He's 'daddy' so he won't lie to them. Anyone that didn't support him was labeled as a bad person. There were days my kids came home and wouldn't look at me, even though I have never mistreated a child.
My wife has told me to be patient. And I will be. I love those kids and even if they never call me 'dad' I will love them as my own. But forum, please tell me it gets better. Please tell me that the manipulative POS Dad gets found out someday. Please tell me my kids won't grow up thinking that manipulation is part of a healthy relationship.
Sorry they arent your kids. They have a dad already. It will probably not get better as you already have formed a bad opinion of the childrens father and no doubt he is as bad as you and treats you the same way. Will it get better? Maybe if you both can get on amicably and both not speak or run each other down in front of kids. If he really is that bad then kids will figure it out themselves when they are older. Things will be a bit easier if your partners kids think you like their dad and want to get on with him or at least trying to. Theres always 2 sides to the stories isnt there .
To put im bluntly, you will never be their Dad. That will always be a fact. The only thing you can do is be the best "step dad" you can be for them. If their dad is being manipulative, he will get found out soon enough. I am not sure how old the kids are, but as they get older and wiser, they will see who their dad really is. When they are in their teens and their dad tries to bad mouth you, they will know you well enough to know that what he is saying is not true.
I had a step son, he would tell me I was the "best of all his mums boyfriends" he never called me dad though, I didnt expect him to either.