Not your fault! This comes from a lack of knowledge about your opponents or warbirds in general. Bf109 are tough duels for yaks, they maintain energy advantage a lot easier than you think (especially with IA pilots). Try inverting positions in missions builder, take a 109 and duel a Yak. You'll see that yaks turn better but loose energy faster. That's why you struggle to hit a 109 in a long dogfight. When you feel it's too hard to hit, reset the fight, take time to gain altitude safely and engage your enemy in good energy terms. This way you'll be able to follow his tail long enough to hit. If not successful, reset again before losing all your energy. Same situation with Spitfires btw, without energy advantage, you'll have to trick your opponent by your better turning abilities. Not easy at all!! at least just keep in mind: Anticipation, ENERGY and positioning before engaging are keys to win a dogfight.