I know exactly how you feel. I've found that the only thing we can do is find ways to cope. I have been searching with trial and error for something that works, but I hate to say I haven't struck gold yet. I have four sets of episodes: depressive, manic, rage, and "mixed" (which usually leads to complete panic and desperation within a matter of minutes). I have some help to share.
I have made progress for coping with my depressive episodes. I've found that for the depression, the only thing I can do is "bat down the hatches and hold on." That's what I call it when you turn your phone off (not for real just don't take any calls), retreat within yourself, only interacting with others when absolutely necessary. But here's the key - you can't retreat into the bottomless sorrow inside because we know where that leads. So when you do this, you have to FOCUS on loving yourself like you would a scared child. This will sound weird but sometimes taking a warm, dimly lit bath or getting in a jacuzzi helps soothe your mind, and you can FOCUS on just hugging and holding yourself in your mind.
Two more things: 1. NO ONE UNDERSTANDS. No one but us. My friends constantly tell me they can't imagine what it's like. And they can't. They don't know the fear, the god-awful loneliness, the dispair. So try to remind yourself that reaching out for help can be a mess. People can't help even though they try, and most of the time (I) end up arguing with them. Reaching in, and learning to find that warm place inside your heart or spirit or mind, that smells like fresh baked cookies and sounds like the ocean and is completely your very own sanctuary and yours alone, is the best way to cope. Since it's like going through a thunderstorm anyway, (fIguratively) let it rain outside and go inside for games and hot cocoa. It may be strange to recommend a bpd to retreat instead of reach out for help, but this works for me.
and 2. REMAIN AWARE THAT THIS FEELING IS CHEMICAL, IT IS CAUSED BY A CHEMICAL IMBALANCE, AND IT IS TEMPORARY! IT WILL NOT LAST FOREVER! This is the most critical thought in my existence. It helps me make it through the down times.
Now if only I could conquer the blinding rage. Baby steps. Take care!