Sometimes she thinks she's just wasting her time.
Those are the times she did poorly on an exam, or her brain hurts from listening to Tosh's hundred miles per hour lectures, or when Owen calls her a stupid chav. Those are the days where it feels like her entire plan is just the wishful dreams of a child, because she knows it's impossible, because if the Doctor couldn't figure out a way, then how could she?
And then she remembers all the things he thought were impossible that really happened, and she remembers that she absorbed the Time Vortex and survived, and did countless other things that other people could only dream about doing, and then it gets easier.
It turns out that she learned more from the Doctor's technobabble than she thought, because soon enough she's graduating with a degree in astrophysics, and she imagines the Doctor's there, beaming at her with pride, and after that, things don't seem so hopeless.
Of course, there's still the problem of the dimension cannon not working. No matter how much she and Tosh and Mickey work with it, they can't get any farther than Glasgow, and that's still in this dimension. After the Battle of Canary Wharf, whatever technology her Dad's Torchwood had invented stopped functioning, and couldn't be restarted. Those were the days things almost looked hopeless again, and baby Tony was the only one who could put a smile on her face.
Then the stars started going out. No one could explain how or why, but what they did know is that it wasn't just happening in their dimension: it was happening in all of them, and suddenly the dimension cannon started working again.
And Rose knew that it couldn't be used for her own gain anymore, that it couldn't just be about her getting back to the man she could finally admit she loved, that it had to be about saving not just the world, but all of reality. And as she tested the dimension cannon, desperately searching for her world, the Doctor's world, she realized how selfish it would've been to use it, to rip apart dimensions and destroy worlds for her own personal gain.
But when finally, finally, she sees his face again, first on a screen at Donna's home and then from down the street, in person, none of that matters anymore because he's there, and she's with him, and nothing will ever keep them apart again.