Also do not listen to salty forum users that spend their time flaming the System and giving you sarcastic replies while they could spend that time to improve themselves but they do not want it.
Mistakes that I have noticed in my own Ana gameplay (please do feel free to ignore the ones that do not apply to you in particular) are: using the cooldowns toe early without getting any value out of them leaving me helpless for the next 10 seconds and only surviving (that is if I actually survive) if the enemy is worse than me and does not have any awareness whatsoever that make them use those mistakes against me.; sleeping targets that my team cannot follow up on, failing my sleeps on targets that would turn the fight in my team’s favour; using my nades either too late or too early, making us lose the fight; keeping my nano for too long for the Genji that is building his blade enormously slow; keeping the same nade for a Genji that has his Blade but does not use it because either he failed to recognize the opening that would give his blade advantage or he fed and died 3 times in a row while having his Blade. Those are currently the biggest mistakes that I can come up with from the top of my head
I am pretty sure that there are more that I cannot remember at this particular moment.
Also, irregardless of what Platform you are playing on you have every right to pat yourself on the back for achieving something you have been working on for a very long time, executing it successfully, just like you should slap yourself on the back of your head for making a mistake, especially the most blatant one.
Another thing, do not only look at replays when you lose, also analyze the replays where you have won. You will notice more mistakes there, ironically, because people get way too cocky if they know that they are winning and would make certain plays that they would never do if they were losing. Those might prove more useful than not unless you are actually smurfing and playing 2-3 elo’s below your main and you are doing everything you are doing on purpose, but let me tell you no matter how cocky you will get, even a Bronze widow is going to hit her shots eventually, and when she does it is mostly those game winning shots 
Do not blame your feeding Hog or Ball for you dying. You decided yourself to get into a position where you willingly risked you getting sniped by the enemy hitscan to heal their behind (actually this one is more intended to berate myself rather than you personally
but it does bear meaning). If they are in a position that in order to heal them would expose you to the enemy hitscan, let them die. I have learned that the hard way. If your team is feeding and for whatever reason do not listen to your callouts or decide to ignore them, let them die. Your life as a support is much more valuable than their feeding behind.
Better to let them die fast and you giving yourself the chance to retreat and live and regroup with them has more value than you trying to keep them alive where you will die first, them second then none of you respawn at the same time, then again they go in alone, making the teamfight 4vs6 and again the process repeats.
And never ever trust your team to finish a low HP target even if you call it out because they are too busy with the enemy Tank who is full HP and also being pocketed by a Mercy and at the same time are yelling in VC “healers you suck”, “help me” while they keep pushing forward while anti
Always try to secure low hp kills yourself and then heal. People in low ranks only see what is directly on their screen, they either do not know or will not turn around to see what is happening behind them.
When I play Ana and my other support is playing Mercy in 90% of the cases I know that she will only healbot so I use it to secure those 20 hp low enemy target kills since her yellow beam is going to be way up our Rein’s full HP behind.
And last but not least, never ever trust your shield. Make a pillar or cover your shield and this applies especially when your team is running 2 OT. IN ML7’s unranked to GM there was this particular match where he played on Nepal (cannot exactly remember right now what that particular map is called), but it was not Sanctum definitely, yes it was Village where he hugged this pillar that was right next to the point and he was like “see this chat, this is my Orisa shield, I am not moving from this spot.” Never underestimate the power of natural cover 