My shower has a wall mount single lever faucet similar to this: http://www.cn-faucet.com/Wall-Mount-Single-Lever-Faucets-141.html
Over time, it has become more sensitive and now, in most angles, gravity pulls and turns it off, unless water flow is very high. If I turn it to get very hot or very cold water, it becomes less affected, and at only hot or only cold, (when it's almost horizontal), it can stay on with a very slight water flow.
The lever has a screw that holds it to the faucet, and I haven't found anything else to touch.
I use it now without the lever, and it's quite steady (hard to use with the hands, indeed; I usually put the lever to turn it on or change water temp, and then put the lever aside).
What could be the problem, and how could I fix it?
Update:
The faucet itself:
The insides of the lever:
You can see the screw at the bottom of the lever; this screw bites the black plastic piece of the faucet to keep it in place; I don't think this is related to my problem, though.
In the center of said black plastic piece there's a screwhole, but there's no screw in it.
If a more detailed picture of some part is needed just ask for it.
Best Answer
It sounds to me like your diverter valve (the round piece w/ the green on it in your pictures) is failing. It may be as simple a fix as pulling the valve and replacing the gaskets (O rings, probably) and reinserting the valve, or alternatively just replacing the diverter valve entirely.
Unscrew the chrome cap surrounding the diverter valve and the valve itself should pull right out. Take that to a plumbing supply shop/lowes/home depot/someone who carries your brand of faucet.