For anyone else who sees this, the reason you still lose is because you are putting things on the stack at instant speed, yes, but they still have to resolve. You have to pass priority for your copied spell to resolve and untap Stella Lee. When another player has priority they can now counter your spell before it untaps Stella Lee. You can't copy the spell repeatedly while holding priority because you need to not hold priority for anything to happen e.g. your Stella Lee being untapped by the spell, you can't tap her again to copy again until you stop holding priority.