Sorry about the lack of clarity, yes, I am playing GNB and was tanking in the static. I had this in my original post but had to trim it for length.

Thank you both for your comments. Just to reply to a few things:
-My overall iLVL is now 520. I wasn't aware I could buy/aug a 520 weapon and no one suggested it to me.
-I put several hours into learning each fight beforehand, including watching the specific strategy videos suggested, talking through mechs with the other members, and watching additional videos to see how other people were doing it.
-For my rotation, I am aware that keeping your GCD rolling is #1 priority, and I do my best to always try to do that and maintain uptime during mechanics. I rarely fat finger a combo, but since GNB is more of a priority based job I certainly make the wrong decisions sometimes, due to lack of experience or panicking if a mech is going badly.

At the end of the day I do understand that this isn't the end of the world, and I certainly have a lot of room for improvement. I guess I should just accept that I will never really know the reason and move on.