Until you get better mechanically, I suggest you start noticing your positioning choices and watch killcam religiously. It will be super cringy the first few times (it will still be cringy later but not as much T_T) but you have to understand what the enemy saw and why they killed you before you could kill them.
If you want to get better mechanically while getting better at positioning, but you don't want the pressure of having to be in the killfeed constantly, I would suggest you try out Zen. Just put the healing orb on the characters who are pushing forward and put the discord orb on whoever your team is focusing. Once you do this, you can focus on trying to land your shots while staying in the backline, in a position where the enemy team has the most difficulties reaching you. If they have snipers, play corners, if they have flankers, try to stay in the middle of your team. You don't have mobility, so you either discover that you are a mechanical god (in that case you'll be in top500 in no time), or you will be forced to learn how to position yourself so that you stay alive as long as possible.
If you want to, you can also try to practise dying as a dps: go Soldier, get to a highground with some kind of door or cover, and enjoy how easy it is to click heads from above until you're physically forced to get down from there. You also have heal and a button to run the fuck away if the enemy starts looking at you angrily. But the most important thing as Soldier is not getting kills, is applying consistent pressure from the top. The kills will come after a bit of practise.
Once you died in a thousand different ways and are pretty good at predicting damage, go Zarya and bubble the shit out of your teammates. You hear an angry Doomfist slamming in the backline? Bubble the Zen and for good measure solo grav the Doom, so he will think twice about trying again. A couple of good placed bubbles will have you at like 80 charge, at which point it doesn't matter how mechanically skilled you are since the enemy team will scatter as you walk forward.
As someone with 1000+ hours on support who plays Widow and Cree in deathmatch while waiting in queue, let me tell you one secret: you are mechanically skilled. My guess is that you can point and click on whatever there is on the screen, be the Battle.net icon on your desktop, the play button on YouTube, you know where they are, you know that they are there, and you can move your muose to click on them without problems. The reason you can't click heads in game is that heads move, and can click back, so you panic, and you shoot semi-randomly hoping something will eventually be hit.
Get comfortable with dying, get comfortable with positioning so that the enemy has the hardest time possible clicking on your head, get comfortable with positioning so that you have the easiest time possible clicking on their heads, and you'll make insane progress in no time.
Source: ex Mercy main, first fps ever, by now I can challenge the idiot dps in my team to 1v1 me and chances are I'll win lol.