I can completely relate. I had that kind of severe, but not constant, depression for a few weeks, ending a few days ago.

What might've helped me is 5-HTP (you can never be sure: it could've been a coincidence), hydroxytryptophan, a serotonin precursor which is a bit like a shock therapy, since both 5-HTP metabolism (the rate/speed-limiting, slowest one) and serotonin are further metabolised quite quickly. Any atypical antipsychotic, however, retains serotonin longer than is usually the case. I took 150 mg in the evening and the next few days in the morning. Today I took 200 mg, but I haven't taken an antipsychotic today (it's quetiapine).

Another thing that most probably helps is to not try to think happy thoughts but purely objective thoughts: just observe your emotions and your surroundings and think about that (not about people, because you're likely to be subjective: that goes automatically and is warped by your depression). Another thing to focus on is ways to prevent these mood changes from happening again. You can influence that, maybe more so than you think, but now is a good time to make plans. Just assume you can find a solution that makes things at least easier and that you will have the time to execute your plan, which is of course likely, but you won't have much time, so you should prepare yourself and guard yourself for future distractions, during mania (or stability).

Edit:
Ok, I realise the depression I mentioned wasn't nearly that long, but I had a severe depression for two years and I survived, came out a new, different person, felt reborn, quite literally. But I was young and I don't mind the new me, much. Not that much. Maybe.

At any rate, it can take a long time and if it does, you'll develop, have developed, new abilities. For better or worse.