I want to tell her that, since I am a PostDoc researcher I perfectly unterstand the need to sometimes vanish from all other personal connections and just focus on research. I want to tell her that I want to try to support her with this decision and are really happy for her. But at the same time I want her to give us a chance to at least try and see if we can balance the work and our fledling relationship.
Being this direct and honest seems like such a good approach already :P
She seems to have decided already that she needs to focus, but you have some experience to bring to the table, as you have the doctorate yourself. Tell her how you understand that it is demanding and though you did focus at the time (it seems), you think it would be worth trying.
Ultimately, work/life balance is everything. I've mostly focused on my career from the very beginning and though I didn't let go of physical activities, I did let romance slip by most of the time. I'm now a lot better now that I have someone to share the good and bad moments with and she might need that, when the research results take too long or are frustrating, or the interactions she is having there make her unease.
You just have to remove the "but" from this sentence:
I want to tell her that I want to try to support her with this decision and are really happy for her. But at the same time I want her to give us a chance to at least try and see if we can balance the work and our fledling relationship.
It ain't negative at all, just tell her you're happy, you want to continue together and support her through her dream.