I'm a Montrealer too and will be starting Cambridge next year.
There's a couple things to remember when applying outside Canada - Our system is special because of CEGEP. The general rule is that your latest grades trump older grades. That means your CEGEP marks will probably mean much more than high school by the time you apply. Your high school grades will be looked at, but stellar marks at CEGEP will probably convey much greater academic potential.
Also, Cambridge will probably be interested in knowing which subjects you take at CEGEP and may (emphasise the may, 'cause I really don't know) place more weight on classes that were relevent to your chosen subject. I know they do that for A-levels for Brits, so I assume it might be similar.
((Also, allow me to impart some wisdom by saying that CEGEP is an awesome time. Enjoy it. Try new things. That last one in particular I can't stress enough. Unlike high school you finally get the chance to do a bunch of random subjects. Take many. You may stumble upon a field of study you would never have expected to love. I started CEGEP convinced I would do engineering at uni. I ended up doing politics and religion, two subjects I would never have been exposed to if it wasn't for CEGEP. (Ok, enough old person rambling!)))
As for your mental health problems, I would mention it if you need to explain a gap in your grades. It's much better that they can put your academic history into perspective and otherwise you risk just looking like you had bad grades for no good reason. A discreete few sentences in the 'any other info' part of the UCAS form should be enough.
Good luck!