I don't pretend to be an expert on mental health and suicide, but IMO as contrary as it seems, you can't look at anyone's activities or future plans on the day they committed suicide and always see a clear pattern that shows they had a possibility of doing so.
This article talks about the role of impulsivity in suicide and explains very well how a person who had planned for a party in the evening might instead take their life in the afternoon:
Myths of suicide
According to this article, a person can be privately struggling along for months or years with suicidal thoughts and depression but
once they experience some sort of trigger they usually go from suicidal ideation to suicide attempt within ten minutes.
So that's why I think self harm is a possible explanation for Abbie's disappearance.
I can get the citation if anyone is interested, but I also read in another article that suicide is twice as common as homicide. So you are much more likely to die by your own hand than through foul play.
I can't rule out foul play completely either, but I think suicide has to be way up there in terms of possibilities. JMO