Why is it so hard to pull the plug on marriage

We have been together for going on 11 years. That’s 9 years married out of the 11 years. Things were rocky from day one and continued to be since. I can honestly say it’s been me that has made it last this long. He’s given me every reason to give up on him yet I carry on. I’m now at the point to where I am at peace with my decision to divorce him, but the three kids we share and our friendship are the elements in this equation that keep me hanging on. He tells me he is tired of putting up with me just to stay with his kids which is a punch in the gut considering I’ve been a pillar for our family- also the good decision maker, the one who keeps it all going and flowing. He does his part, but the balance is off leaving me with the most weight. Maybe it wouldn’t be so heavy if he weren’t so hard to handle. But perhaps he feels this way about me from his own perspectives. Idk what more I could do to make things better but I’m at a dead end road here. He is too and I think we both know we are at the end. As much as we both know we should part, it still hurts. Why do you think it hurts so much when you both know it’s not going to work no matter how hard you try?