An exceptional video. I have read your post in the past an use the video to reinforce the target selection process.
I had it going pretty well the other day in a F4U1A when we were attacking a base and they were there waiting. I had a shot at Del's P38 but he showed me the skinny side then escaped downward. I chose the next biggest threat a 190 and smoked him. I turned toward the next threat and just before firing POW Del smokes my butt. My question then was should I have been more aggressive on the first threat. If chased after Del, then 190 would have picked me as I saw it. If I concentrate on the biggest threat, and not the kill, I have let the chance go of reducing the numbers. Tough calls.
Hard to tell without seeing any video of it but it sounds like Del turned back in for you as you went for the 190 making him the immediate threat again. That's where SA comes in. ALWAYS be looking around, and I mean ALWAYS! Watch my video again and you'll see even as I'm making an attack run on one plane I am looking around and checking on all the other targets as well. I only focus on my target when I'm getting ready to shoot.
Practice setting an internal alarm clock. When you're flying out to the fight and over enemy territory, set the clock for 5 seconds. Look around, see if there are any enemies around to your left, right, above, below, behind, or in front of you. 5 seconds later your internal alarm should be going off saying "Hey! You should check the area again!" Look around again. Look left, right, above, below, behind and in front for targets. You'll be surprised how many times you suddenly see a plane that wasn't there a second ago.
When in a small fight (1v1, 2v1, 2v2, etc) set the alarm for 2 seconds. Every 2 seconds (max) in a fight you should be checking your 6 and looking around to see if someone else is targeting you. Try not to tunnel vision onto one target. I forget who said it but someone has it in their sig "If you think you're having a 1v1 in the MA, you're SA has failed you."
When in very large furballs, like in my video here, set the internal alarm for .5-1 second. ESPECIALLY when fighting them alone. Everything, yes... literally everything, will be targeting you. Be checking up on ALL targets at ALL TIMES!