"I've been decided"

Solution 1:

"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.

Solution 2:

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.