I think you are...
Opened or closed, I continue to be.
Accounts used to track money are opened and closed, but money is not contained or stored in such accounts, they only track/measure it.
A mechanism of choice, But I can be beastly.
We choose to use it as a mechanism for exchange. But it causes us so much grief, we're constantly trying to reform, change, regulate, etc. in order to mitigate the problems it causes us.
I come in all shapes and sizes, Each with a common task.
Cash, and other tokens representing money have many forms. Coins and notes of many shapes and sizes, plastic chips, shells, beads, stuffed toys, cheques, IOUs, etc. The common task is providing a trusted exchange mechanism.
I cling to life by a thread, What am I, I ask?
It is only the fragile trust of the people that keep any currency going. The abstract and virtual nature of money as a resource means that this thin thread can easily break, as we have seen in recent events, and as we continue to see more and more often.
I can be made out of anything from A to Z, and sometimes have eyes with which I can't see.
Anything can be used as a money token, and over the years so many items have been. Some money has people's faces printed on it, the eyes that cannot see, for money is blind as to how it was earned - a drug dealer or a charity worker can take their notes and spend them without anyone knowing how they earned it.