Susan has always collected things. A petrified flower from Skaro, silk pajamas from China, even a giant sugar crystal from when the TARDIS had malfunctioned, but when her grandfather stranded her on earth (she hasn't quite forgiven him for that, and occasionally wishes the same fate on him) she lost her collection of fabulous things.
There isn't much to collect on the devastated planet she must now live on. David brings her flowers sometimes, they wilt quickly and die and she's heartbroken each time. David will wilt, David will die.
In the end, David leaves. Their love is young and desperate and burns out quickly when they both return home tired and hungry and angry. The day he leaves she goes to a empty field and waits for the TARDIS, She cries when it doesn't come and doesn't return to London.
She settles in a small town that was growing because it was the only place in the area that hadn't been destroyed by the Daleks.
She teaches at the school when it opens. The children are frightened and difficult to teach. She collects the good moments. The day a silent orphaned boy finally spoke. When Stewart Smythe, slowly and stuttering, read the first passage from one of the books she'd managed to find.
There is another teacher at the school, older than her. She has no gap in her teeth, and her hair reaches down past her shoulders. Susan takes this anyway and sometimes screams out Barbara's name not quite by mistake.
When they move in together, nobody even blinks.
When tall dashing Ian moves in people raise their eyebrows.
She meets him walking down the road. He is young and fit and looking for his place in the world. (His name is Ian, so she gives him one.)
There are very few elderly people left. Most died within weeks of the Dalek invasion. But an angry young boy who refuses to say his name or tell anyone where he came from is found sleeping on the village green one morning and Susan takes him in too. She teaches him to say please, and thank you. She teaches him to read and write. She teaches him responsibility and how it's important not to abandon people.
They grow old, she doesn't. The boy becomes a man, Ian ages slowly, and the teacher stops teaching. Susan takes long walks and one day just doesn't come back. There's blood on the trail she usually walks and everyone believes she was attacked by something. They never find a body and they're almost relieved.
In another village, far away, a woman shows up and offers to teach.
Susan collects. She collects fragments of memory wrapped in new people. They wilt quickly and die, and she's heartbroken each time.