I would say that this past year has been the hardest on our relationship. My significant other is in his 3rd year, and it's not just a 9-5 job. It's a 7-7. Or a 6a-10p. It depends on the rotation. So I am doing a lot more of the traveling, since my schedule isn't as busy. We try to see each other every other week, or failing that, every 3 weeks. Sometimes I'm lucky and I can come 2 weekends in a row!
(And when I say that my schedule isn't as busy, I mean I'm only in grad school and working full time.) So it does take a toll on me. But I'm glad to see him and to spend time with him.
Granted, we've been together for longer than most people have been married, and it's not the first time in our relationship that one of us has been living in a different city for extended periods of time, but this has absolutely been the crappiest year so far, even though I love my job. Last year I traveled lots but hated my job, but he had more time to be supportive. This year, he literally has less time, and part of me resents that, but part of me understands working 70 hours a week, and that it's my turn to be supportive.
So it's doable, but there has to be a significant amount of trust established. We pretty much don't go a day without at least one phone call (but this is us.) Normal people could probably go a few days. And I know I am really lucky because some of his colleagues don't see their SOs as much, and they live in the same city!
I agree with the previous post about being creative and maybe doing an MPH or research year in her city if this interests you. It's a good way to see her more full-time, and also don't forget that if you guys wind up together, you'll pretty much be asking her to move wherever you match, be it Arkansas to Alaska, and she might not have any job prospects there. also don't forget that if you can make it through M3, you can do away rotations in her city during M4. So give a little to get a little. Our relationship works because at any given point, one of us has been able to be more flexible schedule-wise. I'm prepared to go move to wherever he goes because he's moved for me, and vice versa. It does help that we're getting married next summer before he starts residency though.
For the long term, I think a relationship can only survive if both parties are sacrificing somewhat equivalently or have sacrificed before. But I didn't think of it like this when we were doing it, and I don't think he thought like that either. It's just something I've realized over time that has helped to keep our partnership stable.
Good luck, and stay strong.