Sorry to hear you are going through this sordfish. Unfortunately it’s part of the right of passage. Given an indefinite amount of time you will face many of the common diseases with both fish and coral, be it in a qt or the display, and, it’s scary, frustrating, and will make you want to quit the hobby multiple times, but, what begins to happen over time is the spilled milk story (as a three year old, if you spilled some milk you wanted your disappointment will make you cry, but, 20 years later you’ll just see it a spilled milk and a few minutes of cleanup is all it takes). I myself am 9 years in and still learning (only started with acropora this year and am 0 to 4 so far), and even just recently had to deal with chrysophytes (wait till you encounter that, it made me want to pour bleach in the tank just to be done with it at times), but, persistence, lots of questions, and patience is the key. If you keep at it, 3 years from now, you’ll look at this like a fond memory of you beginning in the hobby.
Good Luck, and, ask questions!!!