How do you stay civil towards a person you have nothing but disgust for?
Could avoid hanging out together or being around each other too long. When it cannot be helped that you are in the same space? Could choose to practice "grocery store polite." Because even the grocery store strangers gets a basic politeness level out of me. "Excuse me." "Thank you." "Please." It doesn't mean I love them or are tight with them or anything. It just means I choose to be civil.
In the case of people I dislike? I can add the rest in the privacy of my own head. *shrug*
"Excuse me (jerkface), I'd like to get the peanut butter."
And how do you turn off caring/loving someone so you can be in a house with them and not break down when they aren't there anymore?
You don't. Feelings are not a faucet that you turn on and off.
Could decide to be ok breaking down at this time. Just let it out in private -- in the shower, in your pillow, with a friend. Wherever is most appropriate and safe.
Could let the feelings do their job. Some emotions are yummy to feel and some are yucky. Internal weather is just that. Weather. Could decide to weather out the stages of grief and let each stage blow over. Could engage in the process of letting it GO even if parts of it feel yucky rather than bottling it up and thus winding up holding on to it.
With the passage of more time, the new normal will become
the new normal, and things will not sting so bad. When you live in separate spaces, not being physically around will also help with detachment and navigating through those stages also. It's just all going to take time it takes.
Hang in there!