Why did she commit suicide?
At the end of the film she destroys the radio and sets off from the beach in the rescue raft, ignoring the rocks that are just a little bit off shore/likely to puncture the raft.
She effectively committed suicide. No one knows where she is, at the beginning they radio in where they think they are but the friend says there's no island there. With today's sat images there are no unknown islands and certainly no unknown island chains.
She could have radioed in that there were still survivors and then set off. Sailing over to that other island probably wouldn't have been any safer than digging in on the island she was on since the other island is within sight of the cannibal island chances are good that there are cannibals on the other one to.