I think you know what needs to happen. So my advice is going to be focused on what you should do knowing this is over.
First: You have to keep your head straight. DO NOT let your emotions get the better of you. You have a task at hand and that is to unwind this situation with a little drama as you can. It doesn't matter if you are sad or angry... or maybe, I don't know... HAPPY this is over. What ever your emotion is, set it aside... this is the dissolution of an informal partnership. If you get angry and set her sh1t outside and get into an altercation with this new dude (because that is what he is... the new chump) that will not get you free and clear any sooner. If you are sad and depressed... you might not be paying attention to what stuff she is taking... or you might not care... If you are happy... well you might have the attitude that you want to give up the store because you're just glad it's over....
Second: Take an inventory of everything... what is yours, what is hers... if you have joint accounts or joint bills, make sure she pays what she owes. If you are moving out... well actually that's better... fresh start if you don't mind moving. Make sure your name is off of everything electric bill, water, lease, if you are renting. If everything is in your name, you have to be cognizant of laws in your jurisdiction. You may live someplace where you can't just toss her out without consequences. She could be considered a defacto 'renter' if she has been paying part of the rent, and she doesn't want to go... you might not be able to force her unless you have a legitimate sub-lease agreement. Especially now... many local jurisdictions have Covid related evictions rules... maybe you don't but it doesn't hurt to check.
Third: Sit down and calmly work all this out with her... remember your job is to unwind from this with as little drama as possible... if you let this turn into an emotional drama... well now you are playing on her field... don't play her game. When she moves out... make sure you are there and stand by the door... I have known men that in situations like this where they just told her get her stuff out when he is at work, only to come home and discover all his clothes were gone. Seriously, one buddy of mine could not figure out why she stole his clothes? Her female 'logic' brain was that, even though he paid for HIS CLOTHES, she picked them out... so they are hers. A few months later he saw her with her new chump, wearing his clothes...
Forth: Understand she had been planning this for awhile. This did not just happen. How all this went down was exactly how she expected it to go, she has all her friends lined up to help her... new dude on the hook. If you want to put her on her heels, get your head straight and move through 1-3 as fast as you can. If you can have that the sit down discussion tomorrow, all he better. She is not expecting you to be clear headed and it will throw off her game.
Good luck, sorry to hear this happened to you, but use it as a learning experience. No live in girlfriends. There should be three things that you rationally evaluate before you get serious with a chick. (1) She has to be financially independent and not an idiot with money. (2) She has to have her sh1t together... flaky @ss habits and behavior can not be tolerated. If she is a mess she's definitely not GF material. (3) She can not have any serious medical or health problems... including MENTAL health. Everything else she can fix, but if she doesn't have these three things going for her... she is BAD NEWS. I don't care how you feel about her you have to set your emotions aside and rationally evaluate her like you would a horse you are planning to purchase.
