Most Helpful Girl
It's called emotional cheating... my daughter did the same thing at one point and as time went by, she developed feelings for the guy, she dropped her fiance, and the guy came to see her from England to Canada... we're not talking a short drive here. Things ended up not working out but the moral of the story is, that guy makes her "think" he's better, more thoughtful, more loving, and more sexually attentive than her current BF... she feels he understands her so much better... and why wouldn't he? Afterall he can backspace and rewrite his reply as many times as it takes until it's sounds just right... can't do that in person huh? So at some point she starts to long his company as opposed to yours... all you do is complain, or hang out with your friends or work too much or don't pay enough attention to her (whatever the case may be) and she starts to wish you were more like him (he's merely an illusion BTW) because he "gets" her and he has time for her and he's always so sweet... and what she fails to see is he's not there in everyday life dealing with stress and daily chores and relationship frustrations... what she does see is.."OMG... he's perfect! The guy I've always dreamed about" Your competing against an illusion who is so much better than you... you can't win.. better men than you have tried and failed... fairytales are always so much better than reality... sorry.
Most Helpful Guy
You are not wrong in freaking out at all. It sounds like she's setting up a sexual activity with him.
Talk to her about it but remain calm.
Let her know that you really love her and want what's good for her... even if it's another guy (its better than her cheating).
Then let her know that if she chooses him, you're gone.
If she is sure the other guy will make her happy, then she should have no reason to hurt you anymore.
What she is doing is cruel and unfair. You don't deserve a girl that has the tendency to do what she is doing.
I'm sorry and good luck bro. Hope things work out.