Thanks for your reply!
SI is a bit of a weird topic in my case, I deleted the details out of the above because I didn't want to be too 'triggering'.
I get flashing images at random times throughout the day, and when I try to sleep - the images are violent, involve me and generally seem pretty irreversible!
Several years ago I overdosed, however - I am glad to say that I had second thoughts (random SMS message from old friend changed my mind) and made myself sick them up - I had unbelievably bad stomach pain for a couple days, but no permanent damage (I know right - something out of a movie).
Since then, several friends succumbed to the urge (2 over the last 18 months), which has furthered my resolve to never do it (seeing what it does first hand, in a sense).
I was convinced it was Major Depression too, but its recently started making me feel incredibly unbalanced (I can deal with feeling down for no discernible reason, but not good/angry/jealous) - I can't see myself continuing successfully (academically/financially) if I don't trust myself, or know my limits.
I don't feel I can trust myself, now that I'm experiencing all these 'new' ... things (I don't want to say symptoms, as I don't know if they are).
I'm on a full bursary for post-graduate study, and my department has me on the list for potentially being excluded for poor attendance (no pressure), because of my difficulty heading to lectures/seminars when I'm irritated or down.
Thanks for your reply

the place I'm going has a fairly horrid historical reputation, I'll admit when I got asked whether I'd go there for assessment I felt a strong flash of fear (for the first time in a long time).
My therapist laughed and assured me she didn't mean
THAT part of the wards (which she phrased as "a haven for those with a more tenuous grasp on reality and their surroundings").
(PS: I also have an honest thing going with my psychologist - I have a friend who basically plays a part when she's seeing hers - which defeats the objective in my opinion!).