I'm so sorry you had this disappointing result. Of course GTD doesn't answer these important questions by itself; instead, it allows you to see your various commitments, which makes it easier to adjust them when moments of change and disruption come. Maybe you now want to add a new project like "Decide whether to change course based on dissertation result," with next actions like "prepare to talk to parents" and "talk to advisor."
If your reference to suicide is even a little serious, please get specialized help right away. You're at a keyboard, so search "suicide prevention" and your area, and resources will come up. You matter.