Things to check:
Ping. If its high then what you’re seeing isn’t actually what is there in front of you (I’d say 100ms+ is unplayable and I usually get 40ms which is decent.) If this is high then I believe there are some guides to settings that can help lower ping for Overwatch.
Hardware: Do you have a ‘profile’ for your mouse whilst playing this game that is different from other games? (I’m assuming it’s MKB and not a pad) A lot of gaming mice create ‘per-game profiles’ so you can change hotkeys etc on the fly.
Sensitivity: Go to training room. Move your cursor slowly between two targets, watch your cursor; can you move it slowly and fluidly in a straight(ish) line from one side to the other OR does it ‘wobble’ a lot? If it wobbles your sens might be too high.
Place the crosshairs on the head of a bot and then step from side to side, keeping the crosshairs on their head. Do you have to keep an iron grip on the mouse to do this? If so, your sense is almost definitely way too high.
Now try ‘snapping’ between two targets. Does the cursor end up ‘overshooting’ where you planned to stop or does it pull up short? If it overshoots, sens is too high. Stopping short means it might be too low.
Although everyone swears by extremely low sens, it’s really ‘whatever works for you’.
I don’t have a rock bottom sens (5) but it works for me. when you first lower sense it can feel like you have to move a lot more to line up a shot, in which case it becomes a case of building muscle memory (as opposed to fighting your mouse for control.)
A few other things:
check your crosshairs and colour settings. These alone won’t suddenly make you an aiming god. But some colours will ‘pop’ more than others on the screen to some people (this is in the absence of issues such a colour blindness.) I found that I aim better with certain types of crosshair and then miss everything with others.
Are your hands cold? Warm them up if this is the case.
Don’t play when you’re tired/hungry. You’ll miss the broad side of a roadhog playing like that.
These are just some things you can try out first. The aiming stuff will either get better or it won’t with practice. When people say ‘practice’, they mean ‘at least half an hour per day of practicing just aiming.’ (some even swear by a minimum of one HOUR per day) I know this ain’t a job so whether you do that much is entirely up to you.
How are you practicing? If you’re yeeting yourself into QP it can work, but expect mixed results.