> ...will I lose my sense of identity?
We tend to think of the word "lose" as having negative connotations. Losing one's sense of identity through zazen is equivalent to disseminating one's delusions. Is that so bad?
> ...will I still be able to "predict" situations so I can be prepared to react to them?
True zen practice is clear vision. Prediction without clear vision is based on a conception of how things are, not how it really is.
> ...he who does not advance, falls behind?
Be mindful of such concerns. Ideas of advancing and falling behind are just that -- ideas. Be present and do not be concerned with attainment or loss.
> ...can I redefine "advance"?
Yes. Just be present, and be mindful of when you start to wander.