I checked your equipment and that's not an equipment issue. You still have 2 wrong items on you: Your Earrings and you wrist.
Wrist: Peridot Bracelet / Aquamarine Bracelet
Ear: Peridot Earrings / Aquamarine Earrings
You can buy those from the market board, just pick the cheaper one, it doesn't really matter.

"i feel like I'm failing as a healer, and that makes me upset because healing is usually the one thing I'm good at in games xD"

I don't know you but from that I assume you lack combat awareness.


I even had to check what boss you have trouble with, but from what I have seen now:
There is 1 AoE everyone gets damage from, you need to heal that, not your fault.

Red Circle spam: If you stand in there, it's your fault.
He flys off and damages 1/3 of the battle field, If you stand in there it's your fault.

You are not playing alone, so no one should take damage from the last two.
If no one takes avoidable damage there should be no issue taking that boss down.