To a certain extent not knowing what you're doing can be helpful. It makes you unpredictable. The second time you know what to expect so you're actually strategizing instead of acting on instinct.
I've definitely noticed in myself that sometimes I get into a groove where I don't really even think about what I'm doing, just reacting to each thing the boss does as it happens and somehow not thinking real hard about it improves performance.
I just lean into it at this point. Round 1 is for getting a feel for the fight. Rounds 2-58 are for trying stuff out to see what works and where I should be aggressive vs cautious.
Because the first go you don't know what's going to happen. You are making split second decisions in real time.
Second go, you hesitant because "his big sword move is coming any second!". Then, still get hit by big sword move because you haven't gotten the timing down. You just think you do and you time it wrong.
Better to not know what's coming and react, than know what is coming and react poorly.
Best guess as to why this happens is that your nerves are on edge the first few times. You don't know what to expect, you're watching the boss very closely because you don't know what they can do, you really want to brag to your friends that you "killed so and so first try".
Even as early as a second try, especially when you got close and have therefore assessed the boss as "easy", you relax a bit and start to think you know what's coming. And that's when you've really got to learn the fight for real.
I was trying to get god apostle yesterday and this was my exact story.
Finally on like the millionth try i got him dead when I power onehit killed him when he was 1/8 life. I was at one sliver of life left and at that point I was fully expecting to die and gave up drinking potions, prepared to start again.
This is why watching your stamina meter is almost more important than the health.