Bear the following in mind:
- Both American multirole lines are relatively high skill lines to play.
- Tier 4 planes will fight in more difficult tier 5 battles more often than not.
The P-43 is not a dog fighter. You're looking to keep high, keep fast and dive either on isolated enemy planes or to drop your bombs on a ground target, then boost back to altitude and go looking for more of the same, perhaps even in a different sector if you're in a busy one. You'll need to learn which planes you can turn after (perhaps a Bf 109 B if it's built for speed rather than manoeuvrability, for instance) and which ones you should pepper with your machine guns and then run away from (Japanese turn fighters, for instance). You can hunt heavies, but be aware your plane is quite fragile and you don't want to let their rear gunners chew you up. Try to stay as part of a group and don't become isolated yourself. When in the bombing phase of your play, be aware of possible ground attackers you can take out, but again be wary of rear gunners (not all have them).
Don't get discouraged. Remind yourself you're flying a plane it takes a lot of tactical nous and discipline to make work.