Uh, this is really weird (for me), but I know someone that looks exactly like her. I thought it was her.
Anyway, the advice that there is definitely another man is dumb. A drop off that abrupt may means she has been hiding her true feelings for the past month/s, it may mean she just doesn't care that much and doesn't mind being fickle about the relationship. As for another guy; there might be, there might not, but I think automatically jumping to that conclusion either shows us that you sort of want there to be one (especially if you texted her the accusation) in the hope that you can be all "fuck that bitch" in order to get over her easier, or that you are just really upset. Unless you have some pretty good evidence I wouldn't listen to your fears of another guy. It's not really an honest way of getting over someone.
The way she acted with you (well, as much as I can tell from one post), and how she just let you do whatever, might indicate that she didn't really feel as strongly towards you as you did to her. She ...
might not know what love is...
Maybe letting you do whatever you wanted and play poker and video games was her way of avoiding you, or not having to mother you.
I don't want to be the devil's advocate here, but it's not always so easy as "Dude, there was another guy, so she can get fucked". People's emotions are complex (especially women am I rite guys) and you wont know until you have a long honest talk about it. Maybe not even then.
If she does come running back to you it still indicates that she doesn't really know how someone should act when they 'love' someone. If she did that to me I would take it as a sign that she can't take me seriously, because you don't play with a guy's heart like that
I guess my advice is to talk to her about it rather than assume things. She might have a great reason that she broke up, and she might want to get back together with you if you talk honestly with her. If you want that. If not, then at least you will know for sure, and that's what closure is; not telling her to fuck off on assumed charges.
As for empathy; yeah I got some. I'm still in love with a woman I was with for two years and I
bawled when I broke up with her - long distance wasn't working, and it was really draining her quality of life.