Not here to guide you on whether or not STPM is for you as many people here have already contributed good advice. However, as someone who has taught for STPM, I can assure you that it's all about hard work. I've seen SPM C/D students turn their grades around and score an A in their STPM examinations. If you do decide to go down the STPM route, I urge you to forget your previous results and just focus. Don't let your previous disappointment haunt you.