You are doing a lot of assuming, and I must say, assuming the best case scenario. What do you envision she could be doing when she's gone on weekends and coming home at 3:30 on other nights? How about assume she's having a full-blown affair, then you won't feel so bad about her moving out. It truly is the best thing for you, if you can stop being weak about it. She needs to see that you are strong. She needs to see that it's fine she's moving out...you'll be okay. Be happy about it. Don't ask her to stay. You might even offer to help get her out (ie "need any help moving out or do you have it covered?") If she doesn't want help...just be gone, maybe for a couple days if possible. YOU be the one to get out a bit.
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer
