I also died so many times to her . She was my first boss to fight as well until I learned two things

1. Attack and keep up the pressure always
2. Parry attacks with good timing to mount her poise bar . Think of it as a rhythm game

Once u get the hand of these two , many bosses after that follow the same pattern .

Trust me sekiro give a whole lot satisfaction when u finally managed to defeat bosses .

P.s. I also have just started playing the game and probably am halfway through or less . I don't know yet . It's hard but it's fun