Just to make sure, the cause of Sakura's death hasn't been revealed, has it?
Not the exact details, but they mentioned an accident a couple chapters back, so I imagine that she was hit by a car, or something to that effect.
Yeah, I think the ending was a sort of parallel/culmination of Sakura's death and Haruki's feelings. She's finally confronting the accident and coming to terms with her own suicidal thoughts.
Overall one of the most profound manga I've read on this site. The only other stories I'm aware of that touch on this subject so well are Murasakino's various works and "Ephemeral Spelling With You." I think it did a great job of discussing the stages of grief, how a support system can help, and finally acceptance.
The last bit about instead of moving on, she went back? Beautiful stuff.