OP, I am sorry you are in this situation. I wanted to post because my situation used to be a bit similar to yours - there were no other kids involved, but I moved in with a guy (rental property) who was not my daughter's dad. He was nice to her but the situation didn't make her particularly happy - she didn't like him that much - and as the higher earner I ended up paying the majority of the bills.....
Eventually I thought, if this isn't actively improving my daughter's life, why am I doing it? So I got together enough money to put down a deposit on a flat for my daughter and me, and I moved out.
To be honest I assumed that my partner would have no interest in me if he was no longer getting heavily subsidised accommodation.....
But after some time apart, we have recently started seeing each other again, and I have to say it's lovely. I love having him as my boyfriend (I only see him when my daughter is at her dad's) but still having my own little separate family unit, my own place, my own life!
He was over for Easter weekend as my daughter was at her dad's, and it was lovely: takeaway Friday night; we went to a spa on Saturday; Sunday I cooked a nice roast. We never used to have such nice times when we lived together - it was all stress and agro. He is actually way more respectful to me now. Familiarity breeds contempt, I guess. Maybe not contempt - but it's human nature to take something for granted when it's always available.....
If you ask your DP to move out, you risk losing him. But if you let him stay, you face th certainty, pretty much, of years more miserable drudge.
I know which I would chose.
Out of interest, have you spoken to your DC about it? Do you know what their preference is?
My daughter was clear that she wanted it to be just the two of us.....