Kudos to you for trying your best in this situation, not an easy one by any means. Open lines of communication with your partner and collaborating with bio-mom can help alleviate resentment and pent up frustration.

Gosh, when I first started dating after my divorce I wanted to date a man with kids as he would be more understanding of what it means to be a parent and put the needs of the children first. Instead, I met someone who was never married with no kids. While we are having issues of acceptance from my kids, I thank my stars that I don't have to worry about dealing with more kids.