The spiritual idea that you are 'not the doer of your actions' is perhaps the single most dangerous spiritual belief.
The thing is, we do not know if this is true, but can easily come to believe it is.
The paradox is, it may actually be true!
If there is an 'Absolute Reality', beyond our current day-to-day existence, then, yes everything may already be decided.
The teaching of 'You Are Not The Doer' is a very attractive proposition to the ego because is offers a 'spiritual excuse' to carry on behaving selfishly and harmfully.