Until I went into the army, I (had been / used to be / was ) an introverted person. And after I I finished army service, I became more confident.
I think all of these are possible.
As for had been, I'm referring to the state of me being introverted before the accident that happened in the past. So, I think I can use had been.
And at the same time I feel I can use used to be and was.. If I don't want to emphasize the time gap between two accident, I feel I don't need to use past perfect. So I think this problem depends on how I'm going to say something.
which one is preferable? and what is the difference?