Darkness before dawn

I have suffered from depression since my teens. I have spent a lot of energy pretending everything's fine, that the periods of utter despair, crying for hours on end, and sleeplessness are nothing to worry about. I don't use this as an excuse, mostly because very few people have any idea what's going on. Outwardly I seem fine, I have passed exams, graduated uni, held down jobs all the while feeling like I'm shattered into pieces on the inside.
I don't know what feeling happy really is, there's just this incessant darkness. Sometimes it lessens and I function better then. I used to describe it as 'falling down the rabbithole', because I don't know how else to describe the feeling of approaching sadness, bleakness.
I am not medicated. I don't want drugs to control my emotions, I fear being turned into a blank slate, unable to truly feel anything, and without a shred of creativity left. An automaton.
There are days when getting out of bed is all but impossible, when I break down, when I feel completely numbed.
There are other days, when it is possible to go to work, to go to class, to spend time among people without feeling like I'm suffocating.
From 17 to 21 was possibly the worst time. I skipped a lot of school, didn't too very well in my A Levels, sometimes I'm still amazed I got into uni at all, let alone completed my degree. But I did. Because some part of me really wanted to, to fight back, to be strong, to push the darkness away.
It comes in waves, like right now. I can feel it closing in, feel close to curling up and shutting the world out. Stress encourages it. I'm stressed about my job, I still have no start date, no idea what's going on, no money. I don't know what's going on with my Masters course, there's a paranoid part of me that imagines I didn't actually get in, that I'm the one that's got it all wrong, and the reason nothing (booklists, term dates) has appeared is because I'm not actually expected there in September. The darkness feeds on this stress and worry, it makes it stronger.
I don't ask for sympathy, I know everyone struggles, I know there are people fighting far harder battles than mine. I want to keep fighting though, I don't want to let the darkness wash over me again. It crushes everything in it's path.
If my posting is a little (more) sporadic, it's because I'm trying to deal with these issues, if I can sort out the things making me worry, I'm hoping I can fend off the rest for now. One day I know I'm going to have to decide how I want this to be. I can't live my whole life feeling like this. It holds me back, it's draining, and I'm sick of being tired all the time.
Please bear with me.