Sometimes, the temporary bouts of depression can be a precursor to a more intense depressed mood. Honestly, if you tell a medical professional what you've written on here, at least in my experience, I think they'd have to take you seriously. They probably ask you if you had a plan and how long you've had it.
I understand caring about other people a lot and holding on for them. It's very true that after a while, that doesn't work anymore. It's a great band-aid to the problem but it won't stick. Life gets overwhelming, and then add some MI to go with it, and what more can you expect. No wonder you can't focus in school or attend work. When I was in my final year of high school, I dealt with this real bad. I eventually dropped out because I just couldn't go anymore. I only held onto my job a few months after that and ended up leaving that as well. After that, I was finally talked into getting some help. One thing I've noticed, like every time before, I can't bother to actually do all of this for myself. That's not going to fix the problem. A person has to learn to do things for them because they deserve it. You deserve to get better. Not live in misery like this. You deserve a chance at life, a good life.

Is there a hotline in your district? I see there are multiple numbers for multiple parts of Canada. I encourage you to call; even if you don't consider yourself immediately suicidal, you should call them anyway.