line 13

You must have a line 13. It is one of those numbers that calls you out. Anyway ...
Sunshine wasn't something Viola could dwell on. It wouldn't matter if she felt the sun on her back this Spring. What mattered was that they were not yet being shelled; they were not yet smelling cordite.
Now we are in the war. This works OK as far as I can see. There is something slightly impersonal about the voice again. It is like when there is a personal quality, the language becomes too contemporary. So to find a heroine - or to write a heroine - of a time, I can't quite make the voice work. But it isn't too bad. I don't mind the contrast between what used to be important, and what now (and I guess artificially) is important. She is perhaps too much a woman in danger - a sacrifical lamb so to speak. So she has to fight off that assumption soonish. But we know she knows about war; she knows the smell of cordite. So she has survived the war, and not only because she has been behind the lines.
She is not particularly likable at this point, we have nothing really to latch onto yet. But I guess this is the problem with the line by line approach. No reader would read this slowly; most readers would already be at a pointy piece of her personality (alliteration again, not happy). But if you can find likability early, and gently, I think it has to be a good thing. Perhaps she needs more yearning in those sentences ... like
She could only just remember how the early spring sun could melt the winter frost from her bones. It was, once, a marker in her year. This year, it had no place. What mattered this year was ... And then the rest of that sentence above could finish here. I like the 'once'.