Hi,
the thoughts you described at the top of the thread are very similar to some that I had as a teenager. (jumping of a bridge when driving on one, jumping in front of a subway while waiting, when driving heading toward a big truck...) The therapist I saw then told me those sudden quick "impulsive" thought were part of my OCD, they were not suicidal thought. Someone else might have some other ideas on that, but I would encourage you to check with you therapist to see if it falls under OCD or not. If it is, CBT or another behavioral with exposure therapy would help.
I might be way of track here, but that was my experience.
Take care