Poor sitting posture has become an increasingly widespread issue among students and office workers, contributing to chronic back, neck, and shoulder pain as more people spend extended hours seated each day. To address this, SitWell-Posture was developed as a posture-monitoring system that pairs pressure-sensor pads with a mobile application built in Flutter and Dart, providing real-time feedback through both haptic vibration and in-app notifications. The system relies on a personalized calibration process, threshold-based slouch detection, and a session tracking and insights component to help users understand their posture habits over time. Key challenges included accounting for individual differences in body type, tuning detection sensitivity, and presenting posture data in a user-friendly format. Two experiments evaluated the system's reliability across different body weights and its resistance to false alerts during ordinary movement, achieving 87.5% and 100% accuracy respectively. These results support SitWell-Posture as an effective solution for helping users maintain good posture in their everyday lives.