Divorced father, need assistance dealing with 6yo daughters behavioral issues, butting heads with child's Mother

I'm a single father who watches my 2 kids 5/6 nights out of the week. Their mother is still around but she works graveyard shift so she doesn't see them a lot, and when she does it's only for a day or two, and then I pick them up from daycare on my day to get them back.The problem right now is my daughters behavior at daycare/ymca. She won't sit down, and when she does she's up with in a minute or two trying to go do something else. After being told to go sit back down or when she gets told to do something she will tell them "no!" and she'll do what she was doing. I've talked to her before about her behavior that she can't keep acting like this, and that she doesn't act this way at home and she sure isn't going to act like it there. She acted this way in kindergarten during the school year, and we would get reports from the teacher saying that she's doing these types of thing. And we would try to discipline her, and she would get better for a short bit, and then she would start doing those things again. Due to her behavior, of the 5 field trips they had, she was only able to go on 1, and that was at the beginning of the year.I was told yesterday by the instructors at the YMCA, that if my daughter keeps this kind of behavior up that they're going to have to expel her from the program. I was upset, disappointed, and many things at the same time. I think it's the most I've been angry at my daughter. I know that she can act good, and I know what she's capable of because I've seen her do it. She can be good when she wants to, but she doesn't. And I feel like it's my fault in somehow, and my reactions to their behaviors isn't helping, and that's something I'm working on.I have ADHD, and I take medicine for it. I have for a year or so now. It has made a complete difference in my work ethic and the way I handle things that require me to stay focused. I believe my child has it also. I mentioned it to their mother before and she is completely against it. I don't know how to get her to understand why I think the medicine could be a good thing for our daughter, she was there to see the complete 180 it did for me. Our daughters behavior is causing stress in our lives with her and in general. and I'm looking for help on what to do. I need to help my daughter in someway, I don't care if it's medicine or some type of other thing, but I need help and I don't know what to do and it hurts.Please /r/Parenting , what can I do to help with my daughters behavior issues? I just want her to be happy and to be able to do the things I know she's capable of doing. She's struggling with these simple issues and I don't want her to fight them into her adulthood like I did.Thank you via /r/Parenting http://ift.tt/28SVCQw