Here's what I now think about Aerith and her future death.
As a Cetra, she's characterised by an ability to talk to the Planet, and since the Planet determines its own fate, she has some ability to see the future. She has foreseen her death. However, she has chosen to regard fate as merely a set of possibilities, not 'set in stone'. The death she has forseen is something that might happen to her. We repeatedly hear her saying that even if you think an event is unavoidable, you have to do everything in your power to fight it. For example, she seems to "know" that the plate will fall, but she is absolutely 100% determined to regard that fate as merely a possibility, which her friends might be able to overturn.
So when she goes alone to the Forgotten Capital, she doesn't go as a willing sacrifice. She goes determined to cheat fate, and believing that she can. Forewarned is forearmed, after all. She goes believing she's strong enough to defeat Sephiroth.
However, she's wrong.