Something like this happened to me years ago. I was in a LTR, became indifferent, gf started seeing a "friend." Right. That's the OLDEST play in the book, btw.
I dumped her and gave her 2 weeks to move out. I was very calm, cool, and unemotional.
I honestly didn't care that much because we were so disjointed anyway, living like roommates, not bf/gf.
A mutual friend helped her move out, I didn't lift a finger. All went well.
I remember sitting and having dinner about 3 days later. It felt like a 600# lowland gorilla was off my back. Blood pressure was maybe 70/20, lol. Felt so good to have all the stupid relationship tension and bs gone for good.
I went NC. To this day, I don't talk to her. Ever.
Good luck.