Call me negative but I don't see this ending as a hopeful one at all...
She's still more or less still in the same situation after all. And though she ended the last page with a somewhat positive mindset that very mindset can go away just like that after another minor set-back.
Cause it isn't
No humans can swing their personality so easily. Even if they can, scars will remain. I can see how Sumi isn't like in a better position then the beginning of the story itself.
Heck it doesn't even talk about 'Hope' or anything, it's simply that 'something happened' and she is 'happy' for that something.
Maybe she will go into despair, or she will have hopeful life, it doesn't matter, they are all 'ifs'.
She considered both the fact that she may now lead a happy life, or have hurdles, but still, she is happy for anything that happened in her life, and it's not stagnant as she thought.
This is an open ending, you can theorize whatever may happen to her after the ending.
Personally, I liked the ending as it wasn't some over the roof ending as if she changed and goes out with her friends or become cheerful, it's just her progress in life being the same, which makes the ending 'realistic'.