Cure; why did it happen, how did it happen and how can I fix it?

I'm sick of this, I'm sick of saying its about perspective. Which is fair enough, life can seem better or worse depending on how you look at it, but I keep looking at the negative side. I can go back to the positive but before I know it, I'm back to the negative. So something must be making me look at the negative side, because it isn't natural for me to be so cynical. 

It's like I keep falling over, I stand up again to run for a couple of steps but I fall over again. The cycle repeats.

I used to be able to run for miles and miles and never fall, I used to be untouchable, invincible, nothing could break my spirit.

So why am I so weak now, why can't I even stand up for more than a few seconds?

I prided my self with my resilience. Now I struggle to keep pieces together, I feel my muscles straining to as I pull each part of me closer together, trying to keep my self as 'one' and not just group of emotions, perspectives, attitudes and personalities.