I wear muffs at the range.
Put in foam ear plus as well. This can make a huge difference.
I took one of my boys to a public range and we just sat in chairs and watched shooters. We wore hearing protection of course, but this can help get you accustomed to random gunfire.
Spend some time dry firing off sandbags. This can really help with target panic and form. You can focus on sight picture, breathing and smooth trigger squeeze. Lack of trigger discipline is a common cause of big groups like this.
I've always found the phrase, "let the trigger surprise you" a little bit of a misnomer. When you are really dialed in with a particular trigger, it's creep and break become ingrained in your muscle memory. You get a pretty good feel for when its going to drop.
Factory triggers can be notorious for being too heavy, too long, not crisp and too much overtravel. When dry firing, does it take effort to pull the trigger? It should be effortless. Even a 4-5 pound trigger can be smooth. This is something you can tell if you, as others have suggested, trade rifles with a known good shooter at the range.
At the range, use front and rear sandbags and do not even get on the trigger until the bags are positioned such that the crosshairs rest on the target X ring.
You want to be holding the rifle, all at rest on target, not pushing it to the bullseye. Good cheek weld with full circle in the scope. If the scope is mounted correctly, you will not have to jigger your head around to get a full scope picture. Then get your breathing right, safety off, and squeeze the trigger. Your non-trigger hand should be securing the rear sandbag or resting on the table.
Not on the rifle.
There is a certain kind zen in shooting for me. Sometimes when you are off, it is better to just pack up and go fishing. Coming to the range carrying stress from family or work will effect
your shooting. Too much caffeine can effect your shooting. Take some time to do whatever you need (drug free

) to dump that stress before you get behind the rifle. I've done burpees, run in place, jumping jacks, etc. Go to the pistol range first and get your ya ya's before you go to the rifle range.
On a side note, it took me some time to get my shooting back where I expected it to be after I quit dipping snuff a few years ago.
This will sound weird, but I've used it to teach pistol shooting. Make sure the range is clear, then go through your target alignment, before you start to squeeze the trigger, hold that aim point and close your eyes. Focus entirely on breathing and trigger squeeze and make the shot. You will automatically stop fighting the crosshair placement. This technique helps immensely with flinching as well.