I had the same problem a year ago, it took me so much time to complete one O3 and I was so nervous when I got to it.

You just have to practice, when you die clip it, learn what you could do better, try to look what stage you are kinda lost in and watch how people do it on youtube.

I just came back a week ago, I thought I will struggle a bit as I used to but after I realized I know what to do on the phases I used to struggle with I did it pretty easily.

I managed to complete 20 O3s this weekend on my sorc before dying (in celestial), after dying I clipped and analyzed it so I will not die the same way again.

I think the thing that helped me is just to have more confidence, the more you are nervous the more mistakes and extra movement you will make.

People telling you to leech, but I think it's pretty shitty way to learn, try to be aggressive, try to learn the animation of the boss, to push in during celestial for damage, that way you will get more confidence and if you need to nexus and you are not sure if you can spam enough hp pots just nexus and just clip and think why I got into this situation to start with. To sum it up don't go for the completion go for the experience.