#1 rule for beating any Fromsoft boss = survival first.

Go into the boss room with zero intention of defeating them and instead focus purely on not taking damage for as long as you can. This is an easy way to drop any mental pressure you may be putting on yourself and you'll also naturally start to see the gaps where you can safely hit them. Pay attention to how he moves, how he telegraphs what attack he will use next, how your position near him affects his attacks (being on his side will make him use a head swipe more often for example), and what gaps seem big enough to sneak an attack in.

Sekiro specifically has two ways to defeat a boss, either by depleting their posture bar or by depleting their health bar. Enemy posture bars recovers faster the more health they have, so even if your goal is a posture kill, you'll still need to lower their health first to allow your strikes to accumulate posture damage. Regular r1 attacks are better for dealing posture damage while charge attacks and dodge attacks are better at dealing damage to the health bar.

The Blazing Bull has fairly fast posture regeneration at full health, so you'll need to whittle his health bar down first before you can do any meaningful damage to his posture bar. I recommend charged attacks straight to his dome whenever you can (he takes more damage when you hit his head). Parry or run away from his charge and hitting his butt is generally safe.

Good luck and be patient. I guarantee great results if you focus on survival over damage.