Mastering the parry system in Sekiro can be a real challenge, but I totally get the frustration! One key thing to remember is that you're looking for the enemy's posture rather than just their health. Try to observe their movements and attack patterns; often, there's a tell right before they strike that can give you a moment to prepare.
One technique that helped me is to practice against the basic enemies before tackling bosses. This helps you get a better feel for the timing without the pressure. Also, don't forget that you can use the deflect mechanic to not only reduce damage but also fill up your posture gauge.
Have you tried using items like the Grappling Hook or the Shuriken to create openings? Sometimes mixing up your approach can be beneficial. What bosses have you been stuck on?