How do I deal with my mother?

My mom, is divorced and has had a boyfriend for a year now, though she's always been this way, even with my father whom she was married with for 25 year, it's been recently bothering me to the point where we have very bad arguments and fights. She seems to disregard the fact that she has children and she'll go out of her way to please her partner while walking all over us [her children]. We could be in the bathroom on the toilet and she will literally kick us out because he [her boyfriend] needs to take a shower, she'll run around in a frantic cleaning the house and making his meals and serving him at his beck and call, she'll buy birthday cakes for us and decorate the house and arrange the furniture based on what and how he likes it. She's a completely different person in front of men. I've realized that she lives to please men and she has no self image of her own, she doesn't understand this when I've sat down to talk to her about it (I've lost count) she has no self confidence whatsoever but she's developed this complex attitude of "I come first, and from now on I'm going to think about myself" which is contrary to how she actually acts in regards to men. A couple months ago her bf wanted to cool things off and she tried to overdose on pills, she also did this with my father, and because I'm the oldest (27) I always end up taking care of my little sister, babysitting on weekends when she goes to her sleepovers, etc. I want her to be happy and I don't mind babysitting sometimes so she can have a life too, but she never appreciates it and she'll often just leave and make plans without consulting me and I'll have to put everything aside. She'll try to kick me out once in a while because I fight with her about this, she gets violent and spits and hits, and then comes back to apologize and I know she only does this because without me she would have no one to babysit my sister. I'm 27, have put my life on hold so many times because of her and she'll patronize me on not being married and leaving the house. Someone tell me how to deal with this because I can't take it anymore.