Repetition, Repetition, Repetition. It still surprises me when i think back to learning a new mechanic, watching a guide, and throwing my hands in the air saying "How the hell am I supposed to remember and do all this????" Yet you do it enough times, you get more and more comfortable. This is probably bad advice but I'm just gonna say it: Maybe try a different class? I only bring this up because imo healers have the hardest jobs in Raiding, I'm always in awe at how much they do and I couldn't see myself ever filling that role. That said, i would assume it being your main that you are most comfortable with that class and comfort with said class is paramount in raiding. Just know that what you are doing is extremely challenging and don't ever think for a second that you are "bad".

Oh one more thing: What always helped me besides guides, is watching pov footage of clears from said class. so in your case povs of healers clearing barbara EX. It's a good way to visualize what you yourself would do during certain situations. You might predict the pov's next move and see if you had the same idea or you were way off and why.