What do you think about me letting my daughter believe my partner is her mother?
Why would you want your daughter to believe that your partner is her mother?
It wold be much easier for me to go out, when she thinks I'm out with her mother
It is never right to lie to your children about something as important as who their parent is. You should always be honest and true when it comes to your children so that they will always trust you.
I wasn't thinking about her feelings or the feelings of my partner. All I wanted was easier life.
You should just be honest with your child, even if you do believe this is the best thing to do in her interest. Eventually she'll find out later in life and the other that she is when she does, the more that she will resent you for lying.