What is the meaning of 成器 and how do you use it？
It is a fixed saying, in modern times, it is usually used in negative expressing. i.e. 不成器

我感觉绝望，我就想成器.

People don't use 器 to refer himself, because the criterion is too low, imply that you are not even an ordinary human.

If you really want to use it, use it like this:


他不成器.
一个不成器的家伙.
老话说: 玉不琢不成器, 我要修理修理你.


There is another famous quote from Confucius: 君子不器 which has a totally different meaning, and the view point is not appropriate in modern times.