You've received lots of good advice here so far. It seems to me that you may be coming off as a bit needy. Not many things chase off girls as much as a needy guy. There is a fine line between being a nice guy, and being a doormat. I'm afraid you may have crossed that line at some point. Please don't take that as an insult. It's real easy for nice guys to do. I was the same way until my early 20s.
Asking her not to sleep in the same house as him is reasonable. Begging her not to will get you no where in the long run.
If you sit down and have an honest conversation with her about the situation, I see a couple of different possibilities. Either she tells her mom to butt out, and tells the ex not to come. Or she's gonna do what she wants, and spend the weekend with him. If she decides to stay home while he visits, I don't think you have much of a future together. You MIGHT be able to convince her otherwise via begging/crying/pleading, but that is a dead end road in the long run. Eventually, she's gonna do what she wants anyway, begging would just prolong the misery.
I think you said somewhere you are 20 years old. Were you ever happy with your life before you met this girl? What made you happy? Whatever that is, you need to get back to it. Fishing? Music? Cars? Basically, find a hobby. Just do whatever it is you like to do. Eventually, you will meet some new friends, possibly even the female kind.
Just my .02