Thanks again, majorfailure; you've been extremely helpful.

Heck, you're now the reason I'm still playing, really (see below!).
Well, I'm "good" enough to shoot down a couple enemy aircraft on most missions, now -- as long as my plane is as good as or better than the enemy's. I am, however, absolutely clueless as to how to improve vs. superior machines.
Or mostly-superior. The P-39 might not be superior to the Bf-109 (G6) in
every respect, but I'm having trouble figuring out how to use the few advantages I do possess... namely, climb rate and better throttle performance at altitudes above 5000m. I cannot seem to use climb rate to get away after an attack run; I get shot down every time, no matter if I point my plane near-vertical or do a gentle climbing spiral. I can't just outrun them, since they are just as fast as I am; and as they maneuver better than I do, it seems like I have no viable tactics left if BnZ is untenable!
This leaves me only altitude. Well, I can avoid getting shot down if I refuse to go low, since they refuse to go high; I've been trying to lure them up there, but no dice. It's actually extremely frustrating to me; I nearly gave up on the game altogether, in fact. Just thinking about it makes me mad still, heh. (C'mon, they can out-roll
and out-run me? Haklangr is not perfect?! *throws joystick*)
How is this matchup possible unless P-39 makes a huge mistake?! I know some matches, one must just avoid, but I didn't think this was one of them.
But when I'm tired of tempting myocardial rage-infarctions, I take
majorfailure's advice and have been trying out other Bf (and Fw) models and fighting other enemy aircraft... and it's back to being challenging, fun, and exciting.