I agree with Stoney,..
You're either way too much in love with this girl or a little OCD. Either one isn't good. You probably need some space from her anyway, just so you can stop with the attitude that she's "all" you got. A lot of women retain feelings for ex's. If they act on them, then they don't have enough feelings for the current guy, plain and simple. Although you didn't let her know that you weren't ok with him visiting so it's mostly your fault.
You should have a talk with her. Let her know about your feelings and that you're uncomfortable with having her ex come and stay for a weekend. If she's not interested in letting a friendship with an ex slide, so that she can have a healthy current one, then you need to move on. Say it to her once, don't beg, and be a man about it. Or he might visit eventually and during a weekend when nothing is going on, so she can devote her time to him. One thing leads to another, and the next thing you know she's telling you that she has to explore those feelings, because the relationship never really ended, blah blah blah,..
If you had let her know you didn't want him to visit, and she was still going to let him come, then you're only hurting yourself by hoping for something that isn't as real as you feel it.
As stoney says,.. never be with anyone who is any less crazy for you,.. than you are for them. Chillout your feelings, take time from her if you need,.. and if her's don't develop to the point where she wouldn't want to have an ex come and spend the weekend with her, to avoid making you uncomfortable, then you need to quitely move on.
I had a similar situation develop. She said she had to go explore feelings for the ex, who she always said was the wrong guy. I said go ahead. Two months later she came back and said it was a mistake,.. my response (because we had spent a considerable amount of time together at that point),. was "I was your second choice then and after being together for six months. I don't know what really happened with your ex, but I'm not going to be anyones second choice. There's too many great women in the world and I'll find one who wouldn't pick an ass of an ex over me. Sayonara." She even showed me her awesome boobs to try to change my mind.