I've just started a job in mental health as an AIN, whilst I finish my studies. I hope to work in mental health when I graduate as a RN next year. I also did my mental health prac last year... and TBH, I didn't see one violent incident. It really depends where in mental health you are working. Places like the Psychiatric Evaluation Centre, and High Dependency Unit, and forensic mental health would see a lot more violence than say adolescent or geriatric mental health. (Emergency Depts also see a fair bit of violence...)
Even mental health in the private system, which seems to predominantly deal with things such as PTSD, eating disorders, PND, etc - I can imagine they would see even less violence than on a general medical nursing ward (as these can have violent patients too).
Today I actually completed day 5 of my ABM training (agressive behaviour management). Before I started I was quite sure that I was not assertive enough, and ill equipped to deal with any sort of escalation... but seriously, the training was brilliant!! The first 3 days dealt with general situations, and self defense (which I think is relevant to every nurse or healthcare worker!). And the last 2 days of the training was all about takedowns, restraints, and placing a client in seclusion in the safest manner possible. I can't speak highly enough of the training, it made a huge difference. The trainers helped set up realistic situations, and the hands on experience makes such a difference to your confidence levels.
I'm really appreciative of my new employers LOL, because as soon as I was employed (only 2 weeks now) they put me through the training straight away. So if you do end up with a job in mental health, make sure you push for the training ASAP, as I dont know if all hospitals/instituations put their staff through it as quickly as my hospital did.
Good luck with everything... I felt exactly as you did, and have been really happy that I've stayed true to my aspirations, it been worth it so far