you can't OD on it. You can put a plastic bag over your head and take it to kind of lose consciousness while you suffocate. You'd need to read up on the bag method. Or you could take it in the bath and see if you suffocate. If you don't, then no harm done.
You could just see first if it eases your depression - worth a shot surely! This is the video I watched yesterday:
Saying that, you are super-young so why not try some more meds. They just put me on quietipane and while I was dreading it, it's actually really helpful.
I would try at least 10 meds before CTB. And try the pschedelics too - not necessarily in combo. You couold be a lucky one. If you have finished uni, you have some functionality...
Have you tried other drugs to feel better e.g. weed?