I'm 43 and have been shooting flintlocks since, well, I'm not sure when. Pennsylvania has a flintlock-only deer season, and I know I was shooting for a few years prior to starting to hunt that season at the age of 13.
I spent the VAST majority of those years with a flinch of varying degrees. One of the "ways to cure a flinch" that was used on me was to charge the rifle, then not let me know if the pan was primed or not.
I will 100% recommend against this tactic. It made my anxiety worse and my flinch got much, much, much worse.
It wasn't until I decided I'd had enough of not shooting a flintlock well, and was gonna come to terms with it, that I got to terms with it.
This involved, as Darkhorse mentioned, a substitute "flint". I cut the tail ends off clothspins and used them as replacements for the flint. I put a blue piece of painter's tape on a blank wall, and set about dry firing. Over. And over. And over. My only emphasis was sight picture. That's all I thought about. All I cared about. I didn't about the trigger, lock working, etc. Sight picture. Sight picture. Sight picture.
When I went to the range, that went out the window and the muzzle dropped like a rock off the Empire State Building.
And I realized what was different....firing of the gun. Recoil. I was anticipating it.
Soooo.....
Pull the gun tight...tight....TIGHTER....into my shoulder. More accurately, I pulled in and then also pushed my shoulder into the gun to take a slightly weight-forward posture. Nothing was so tight that it made me white knuckled nor shake/vibrate from tension, but things couldn't be slack.
Then I took a page from shooting compound bows. Back tension is how one releases the trigger on the release when shooting a compound. Pulling the trigger is a flinch or miss in the making. Squeezing the back muscles, pulling the elbow back, and letting the hand's rearward motion bring the "dead" trigger finger along for the ride is what activated the trigger.
So....
Back to the dry firing. Buttstock into my shoulder. Shoulder into the buttstock. Sight picture. Sight picture, sight picture.
Focus on the action of snugging the gun into the shoulder as what subsitutes for "back tension" on the bow. Let that trip the sear.
Dry fire and more dry fire.
Back to the range, now.... safety glasses on. "I can't get anything in my face because I have my glasses on." Repeated that.
Then.... "If I worry and flinch, I KNOW I will miss. It can't be worse than that, so just do what you practiced."
Form. Form. Focus on all that practice you did and replicate that form.
Boom....hit!
Again. Boom...hit!
Again. Again. Again.
Some relapses occurred, but now I knew what a clean shot felt like.
And I did a lot of shooting. Shooting up amounts of powder that would suffice for many PA hunters' lifetimes, and in a single range session.
Now.... I'm far from an expert marksman, but 50 yards offhand, a deer isn't safe by any means. Squirrels aren't, either, although their chances are a bit better than the deer, lol. At 25 yards with my 36-cal schimmel, if I don't get sloppy when the sear breaks, the squirrel dies.
Long way to say.... Practice is important, but it has to be GOOD practice. Be critical and be honest with yourself, and address what's not right. Don't sugar coat anything to yourself. You know what's wrong. Admit it, then attack it. And then you'll fix it.