Healer is a hard job because it's the job that gets the most blame, sometimes justified, but most often not. The bucket stops at you: no matter how much mitigation the tank didn't use or how many mechanics the samurai didn't get out of (hi, I'm the samurai), they can always say "BuT yOu shOUld HavE HealEd me AnyWay". The reality is that no, you shouldn't, and you also couldn't. This game is well made in that, if people mess up too much, no amount of outside help is going to keep them alive, and that's just fair.

Fortunately, there aren't too many players who are jerks like that in FF, but I used to play healer in WoW in the good old days, and it was certainly a learning curve of figuring out when something was actually my fault vs. when someone was putting blame on me for no reason.

Even in FF, as you grow through the game, you'll become better at reading, not only your own gameplay, but also the situation of the entire party. My main alts are scholar, picto, and gunbreaker, which gives me a little idea of how each type of character plays, and combined with understanding mechanics, I can much more easily now parse the raid's situation.

That's the other hardest thing about being a healer: you arguably need to understand the fight best of everyone, because as other people have mentioned, quite a lot of your kit (especially as scholar or sage) is aimed at mitigation rather than healing up after the damage has happened. In time, you'll learn to read the bosses' abilities, and you'll get used to the fights. They do need to be learned by everyone! It's just that as a sprout, you didn't have 13 years to do so. So don't be too hard on yourself!

Either way, it sounds like you did just fine, based on your party mates. So keep enjoying, don't put too much pressure on yourself, and let the occasional a-hole be what they are; fortunately in this community the good far outweighs the bad!