How do I have a hard conversation?

Hi,
So firstly back story. Engaged to my partner of 7 years. When we fell in love we moved straight in together and blended our family of 7 kids. Wow, that wasn’t easy....We have had more than our fair share of ups and downs. Leading to us living separately. We cannot for the life of us communicate about issues. My issue at the moment is I own my own home (mortgaged) and still have kids at home (teenagers). My partner rents a room off a guy but has been staying at my house full time for weeks on end. I need my own space and this has been discussed before but I have no control over when she stays as I suggest I need a night on my own and well she laughs but is still here. Do you think it’s fair that I am starting to feel like if she is here all the time she should financially contribute? I feel like I have another child to support. I cover all expenses including food. I fear hard conversations as in the past these have never gone well. I thought maybe some $ per week if she is staying regularly at least. I can’t commit to her moving in permanently (basically just bringing her things) as my home life is a little crazy. I have teenage girls and older kids. With some mental health stuff etc. My partner isn’t into family stuff and well I am. That’s why there is an arms length from me there.
Help????