You may be looking for a new roommate or a new apartment..... this kind of issue should have been sorted out before adding a new puppy into your shared space, particularly in light of what you describe.
Meet on neutral ground and walk them back together. Pick up any toys or food items beforehand. At home, all interactions should be supervised at all times. Keep a leash on at least one of the dogs so you can grab them if needed. Separate them with baby gates routinely so they can have some alone time. Do not give treats or food to both at the same time, and feed separately.
But as Anna said, this may be a recipe for disaster. Have you ever tried having another dog in the apartment...?
Get a crate and train one of the dogs to stay in it. Never leave them together where they can have at each other when you're not there.
Obedience-train the roommate...oh, sorry, the new dog right now. Do not wait and do not dork around trying to do it without obedience-training classes.
Feed the dogs separately, and see to it that the puppy does not try to raid Kipper's dish, even when he's finished eating -- don't let the pup even lick up any stray molecules from the empty dish or the floor around it. Weirdly this has worked wonders with Cassie and Ruby's competition issues. Train both dogs to "sit" before they get anything: food, treats, doggy-walk, whatever. Make it clear that the humans are the bosses and the dogs have no chance of ever acquiring the throne. This also is surprisingly helpful.
But don't panic until you see what happens. I brought a greyhound into a house occupied by a German shepherd whose dearest, most heartfelt wish was to rid the world of other dogs. We introduced the two, both muzzled, on the sidewalk outside the house. After a brief get-acquainted period, we walked them into the house together (still muzzled). When the Ger shep's homicidal impulse subsided -- quicker than I expected -- we unmuzzled first the grey and then the shepherd. They lived happily ever after as best buddies. The shepherd was female (very dominant) and the grey was male (a cream puff) -- and o'course different dogs behave differently.
And Marie...it's your apartment. It has a door. Practice saying "good-bye" in front of the mirror until you can bring yourself to open the door and say "good-bye" to her face. Roommates are a dime a dozen. Think "Craig's List."