Don't write the new plan until you've understood the old one. Start with a What, So What, Now What session to get the team past the facts of last quarter and into the implications, because the facts on their own just produce defensiveness. Move into a Retrospective to lock the systemic changes in place so the mistakes don't repeat — the difference between a retrospective and a complaint is whether something changes afterwards. Before launching the new plan, run a Pre-mortem — imagine it has already failed, six months from now, and ask why. The optimism bias blinding you this quarter is the same one that blinded you last quarter, and this is the only reliable way to get underneath it.