First off the early war German cannons are IMO the hardest guns in the game to master.
Low ROF, poor ballistics, and at least in my opinion hard to keep on track for a good killing shot.
Maybe its time for a change, force yourself to go back to basics, relearn how to shoot.
Instead of flying one of the harder planes in the set, jump into something thats easier to fly and kill in.
Confidence can be huge, without it nothing works, you die, lose more confidence, slump even worse.
It becomes a self defeating feedback loop. You have to break the chain.
Fly something that is easier to get some kills in, get a couple, get out, land them.
Get that boost that jumps you up and gets you eager to go do it again.
Midwar la5, spit 9, are very good rides with good guns.
Late war Spit 8, N1k, Yak9u are great confidence builders.
I agree with PFactorDave, if you've been flying tracers on, try them for a week off.
Give yourself a half hour in the TA with the lead computing sight to get a good idea of what the right lead looks like.
Then go dive in and shoot till you start hitting and getting kills.