>Girlfriend's moving across the country in a few weeks for university, and we've decided we're doing an open relationship.
Stopped reading there. You've already doomed yourself if you don't just break up cleanly before she leaves. Open relationships, ESPECIALLY if you're the guy, are fucking retard-tier.
Here's exactly what will go down if you go through with this:
>she moves away
>you stay really close for awhile since she'll be adjusting and will need that familiar comfort
>you're still in love with her, so you don't do much but mope about her
>she starts to get more comfortable in her new surroundings; starts to make friends
>eventually she starts to like some guy a bit (and why feel guilty? I mean, we're in an open relationship… my bf said it was cool!)
>"Yeah, Anon, I-I still love you and you're the only one I think about :^)"
>you'll believe it because you feel that you NEED to
>she's fucking this guy (or multiple because loads of chicks become whores when they go off to college)
>B-but she's not like most girls
>yes she is lol
>eventually she stops putting in effort to talk more and more
>you're confused and hurt by this; you get upset with her
>she'll just end the relationship with you and it will be very blunt
>she'll feel bad at first, but she'll get over it soon because she'll be having tons of fun (and cock)
>relationship will be done and you'll be heartbroken and angry - and honestly, it'll be partially your fault for being so naive
I know that I sound like harsh piece of shit, OP, but come on… you're barely out of high school (by the sounds of it). I'm not going to say that "there are other fish in the sea", but the fish that you have on your hook is guaranteed for leaving, not guaranteed to come back and you were young as fuck when you caught it. The chances of you guys staying together long term are astronomically low even if you two had stayed in the same city to begin with; it'll hurt less to break up now than to break up later - and protip: open relationships aren't some porno fantasy where both of you are getting lots of pussy/dick… it's pretty much going to be plenty of dick for her and maybe a fuck or two for you.
You likely won't take my advice because you're young, in love and breaking up is scary because you're changing your world - but your world is going to change very soon and you know this already… might as well take a big hit now than take a bigger hit in a few months when she says that things have run its course. Shit, maybe even you'll find someone new; that's not out of realm of possibility too.
I sound like an asshole, but I'm really just trying to help you, OP. Let us know how things end up in a few months or something.