I have chronic depression. As hectic as that sounds it isn't so bad. It just means I have more than 4 depressive episodes a year (about once a month is accurate) and the episodes aren't as intense as many others experience. I can get by without medication. Being positive can't rid you of all kinds of funk. I am a pretty positive person and there is diddly squat I can do about it when my chemical balance decides to flip belly up. Being positive takes the majority of the edge out of it though. I know why my food tastes bland or why I suddenly just feel like I want to spend the rest of the week in bed. I know that it is worth going out and doing things when I feel like it isn't. I know people care for me deeply when I feel like I am unloved. Being positive makes my depression an inconvenience, but it doesn't take it away.
I agree, Ive had a weak depression (scored a couple points below the threshold for a 'proper' episode), and trying to be positive actually made me feel worse. CBT really helped though. Like, REALLY. Because it's all about being factual and reasonable, rather than positive. Replacing the thought "My life is meaningless and Im just going to feel like this forever" with "My life is awesome and Im growing everyday and everything is perfect" isn't healthy. Its replacing one irrationality with another. But consider the thought "Im in a depression which makes everything feel more difficult than normal, but depression is treatable". That's a more reasonable approach, and it actually lets you give yourself a break :)