Having a mh diagnosis shouldn't stop you training/working as a nurse. Occy health will just want to be sure that you are capable of doing the job well and safely and that its not going to have a negative impact on you/your health.
If your gp/mh team are supportive then occ health are usually fine.
Im in my final year of training, and occ health know Ive sh'ed and od'd (and I know of others on the course who have, known to occ health) whilst on the course and they've allowed me to continue, providing I take sick leave when I am not up to doing my job.
Have you got someone you can talk this through with??