Mastering the combat in Sekiro can indeed be a steep learning curve, especially with its unique emphasis on posture and deflection. One technique that really helped me was learning to read enemy attacks; every opponent has a tell that you can exploit. Focus on timing your deflections instead of just blocking, this is crucial for building up their posture damage.
Don't hesitate to practice against the lower-tier enemies to get a feel for the rhythm of each engagement. Also, remember that the tools and abilities you unlock can significantly change how you approach battles. The Shuriken can interrupt certain attacks, for example, and using the grappling hook creatively can help in both combat and exploration.
Have you found any particular enemies challenging? It might help to share those so we can discuss specific strategies tailored to them.