"I've been decided"

If "I" is intended as the object of "decide", neither the Oxford nor the Cambridge online dictionaries give examples that fit your construction. I can decide something but I cannot be decided, essentially because I am not a decision - that just does not make sense, not even in the sentence "My parents decided on me" (= they decided to have a baby), which is the closest I can get to making myself the object of "decide".

The other meaning is acceptable, if you say something like "I have been decided for some time that I should write a lyric". In this case "decided" describes your state of being.

I hope this helps.

There is no problem in using this construction, it is quite current (ngram and the meaning of the verb "to decide" in this transitive use is verified in the Shorter OED.

• 2 v.t Bring (a person) to a determination or resolution (against, in favour, to do) Early 18th century.

"To have been decided" means that one has accepted to do something but it does not mean that they did it willingly.

"I've been volunteered" is in common usage and also plays off the same theme that you're being forced into doing something, whilst outwardly appearing to have decided to do it yourself.

The idea is that I want to stress that I have not decided to do something; instead, it has been decided without me that I would do this.

It should be the passive form "It has been decided [by someone] that I will do X."

Is it grammatically correct to say "I've been decided"?

No, not in that context.