Advice on custody with ex-wife who is dating her affair partner now

My wife and I are separated since April. We have a 4 year old daughter. She cheated on me for 6 monhs last year and we tried to make it work since D-day in September but she left in April. Now she's with her affair partner. We are still arranging custody for our daughter and doing 50/50 on an interim basis. The trick is my daughter and I are attached to each other but I also have severe kifoscoliosis with severe pain esp after full days I spent with my daughter. My wife wants to have full weekends so that she can dance tango every two weeks where she met her affair partner. But I have severe pain when I'm with my daughter in full days. I have two questions, how do you manage to co-exist with the affair partner? and what would you do if you were in my situation?