It's because we behave according to 'shoulds'.
It's because we make decisions with our heads rather than basing them on feelings, rather than integrating the two.
It's to do with what we were shown, as kids, what an adult relationship should look like, and how our emotions should be dealt with.
What was your parent's relationship like, OP? Loving? Respectful? Did they listen to and care about each other's emotions? What were they like with you? Understanding and curious about your core nature, your actual self?
For me, I was set a poor example by my parents, who felt crap together and fought a lot, but insisted on staying together. As an adult, I've had a lot of relationships where I felt we should stay together, regardless of how shit the relationship made us feel.
Also, with all the fallings out, my feelings were pretty low on the list of priorities, and that's where I've put them for much of my adult life.
Teach your daughters that how they feel is THE most important thing. Teach them that if someone makes them feel bad, that person gets one more chance, in case they did something inadvertently. State once that you don't like the behaviour, and how you feel when they do it. If they keep doing it, leave, because that behaviour is more important to them than you feeling ok.
That's it. I wish someone had taught me, and I hadn't had to go to counselling in my 40s to learn it, but hey ho!
Your girls are so lucky to have you 