It wasn't that she didn't know Neville was brave; he had been Sorted into Gryffindor after all. But when Harry was around, it was like comparing a bright meteor to a small star shining galaxies away.
Then Harry left. Harry left on his noble quest to fight Voldemort, leaving her behind. She knew that she could be just as brave as him, but it seemed that in his eyes she would always be Ron's little sister.
She nursed the flame in her heart for a few months, but soon the harsher reality of life under Snape prevailed. And Neville stepped up and fought and planned and resisted. Best of all, he never questioned that she was good enough to fight. He never tried to leave her behind. They protected each other, as equals. And she realized that maybe she didn't need any flashy bravery to keep her going.
It helped that Neville grew stronger, more self-assured. She saw his occasional forgetfulness as an adorable quirk, rather that as an irritating annoyance.
So when Harry came back after the search for all of the Horcruxes, Ginny was done with Harry. He was brave sure, athletic and kind. But Neville had managed to keep a little sweetness that touched her heart. Ginny realized she didn't need a great hero who rode in to save the day. She just needed someone who would fight for what he believed in, but could still make her laugh at the end of the day. Someone who could be relaxed, instead of intense all the time.
Perhaps Ginny wasn't as brave as she thought she was, but all she knew was that someone a little less damaged was the one for her.