Y has already warned you of the possibility of walking neatly into the trap of wanting to mother Z, because your heart has gone out to her. But may I now also offer you another perspective.
It is good to see you speaking to Z from the angle of your own experience. And the courage you demonstrated in having to make your own difficult de-cisions is truly an example for all who are struggling in their respective relationships. Nonetheless, take care that in wanting to uplift Z you do not inadvertently cause her to fall into the trap of self-pity. Your email to her was a truly wonderful support! Nice and strong and clear! Just keep on making her believe in herself, her strengths, and her deservability.
Take both Y's perspective, as well as my own, as the two possibilities of the growing relationship between you and Z. Try always to be aware of which polarity you are coming from.
With encouragement and warmth,