Oh my gosh!
I we are in the same boat. I think I've been in it a LOT longer than you though. My wife flat out told the councilor that she will not change. After hearing this and knowing my feelings, my therapist told me I would be wasting more of my time by trying to make our relationship work. She wants to work on the marriage but she will not change!
Take it from me, Do NOT stay in your marriage for the sake of your daughter only. You may think it is for her good but it really isn't. She needs a father who is in a happy healthy relationship. It might be different for you but I've spent well over 10 years in a cohabitual relationship and I'm here to tell you it doesn't work. I got to the point that my longings for what I couldn't have were affecting everything in my life. I became LESS of a father. Work became a pain for me. I would sit on the couch and pretty much wallow in my depression. Do you want to go years or maybe decades without love , passion or sex? Well, I have and it is a really bad idea! Now I've wasted all that time and I don't know if I will ever experience those things again.

If you want to work on the relationship then by all means work your butt off! Work to bring back the things that a healthy relationship should have. DON'T stay in your relationship for the sake of your daughter. Even though it may seem like it, It won't be in her best interest. It most definitely will not be in yours! Please don't make the same mistakes I did.
I understand what you are going through and I feel for you. I hope everything works out for you.