The trick is to stop blaming yourself for people dying, sometimes it will be your fault but most of the time 95% of the time someone dies it’s because they ignored a mechanic. Healers can not out heal 1 shot mechanics.
Next thing to consider, healing gets easier the higher the key ( there is a limit though ) the higher the key the better the players will be. When you play with good players they make your job very easy because they are avoiding a lot of extra damage, obviously keys do start getting harder again once gear is no longer getting better but the key difficulty continues to rise, basically around 16-18+.
Lastly remind yourself that you are a healer, you pretty much get instant invites so if you mess up hard and the key falls apart just instantly join a different group and try again, nobody in that new group is going to know you just made a mistake that caused the key to fail. Practice makes perfect.
If you want Discipline Priest specific tips and tricks for mythic+ feel free to msg me in game Synnlawl-BleedingHollow alliance, I am a 3k disc priest who can help you out.