First film, your dive in on the p39 was ok, but could have been better.
Learning to judge closure speed is tough, esp when they down low.
Attacking them when they are that low can be very tough.
When he turned you pulled up which was right, but you should almost instantly roll to keep him in your up view.
And then roll right back in on him. Its faster, doesn't let him open up as much distance between you.
You were doing ok vs the P51, but looks like you didn't see the Jug till he was solid behind you.
You had a choice at that point, he's in guns range, and is about to blow you to pieces.
A go with the defensive barrel roll, and STICK with it till your dead or he's no longer on your 6.
B Go with the break turn, into a turn fight. Since you have a good turn advantage to work with.
You started to go with A, then when that didn't really work, went with B. Well it worked, but you got lucky.
Would have been better, quicker to go with one or the other, and commit to it, and make it work the first time.
Rather than starting a barrel roll, then going into a turn when you didn't shake him. Once you started turning you quickly out turned the P47, and had him in good position. You got a big break when Von showed up and kept the pony busy. Letting you saddle up and kill the P47, nicely done there, and nice shooting.
Overall good throttle work, and if you'd been smart you'd of gone home after those 2 kills and avoided the P39.
But thats hindsight.

In the other film, once F404 in the P51 showed that he intended to just HO and reverse, you need to start making it tougher for him to do so. And set yourself up to get behind him at the same time.
Instead of pointing your nose right at him as he approaches, try sort of an offensive roll, up & around.
Especially if he's climbing back up to meet you.
The idea would be to A make yourself a rolling, moving target so he can't just HO and extend.
B put you "Above" him as he flash's by, letting you go nose down, roll (till you have him in up, or upforward view) and then pull onto his 6.
You need to do it nose down as he has the speed advantage, so by pulling into essentially a split S onto his 6 your boosting your speed enough to make you more of a threat. At the same time your cutting angles off your turn. By going nose low and ROLLING onto his flight path instead of TURNING there. This would if you can do it force him onto the defensive, keep him from more HO passes. You also want to be very aggressive about getting onto his 6. Once there you can force him into mistakes.
If he tries to turn after merge, roll away and under and back into his low 6 position. (sort of a low yo yo like Rolex talks about) Make him think your turning away, when your actually rolling under to attack. He ended up colliding with you, you didn't win that fight, but didn't lose it either.
Dive in on the low p39 was good, not your fault he got killed. Much better approach that time. Lead turns can be tricky, work best if you can get out of his view by getting below his tail. Then attack going uphill (which also helps you control closure speed) Again, as you go up, roll to keep him in view, and get right back on him again. Don't let him get seperation or distance.
On the spit, seemed to me that you were slow turning back into him the first couple merges.
Then you lost him, were looking back and up for him when he was at your high 10 oc. He missed his shot (lucky for you) you recovered nicely and were able to turn and saddle up on him. From there it was just a matter of time till one of those shots nailed him.
But you got lucky!
Now, don't get discouraged, there is a lot of promise showing. But you could work a bit more on taking merges into the vertical, unless you gain something by not doing so. I'd also suggest some practice on avoiding the HO.
Also practice when your bouncing low targets and they turn, so you pull back up, rolling to keep them in your UP view, then transition right back nose low, roll onto their path, and pull onto their 6. Attack attack attack, don't let them have that seperation time to reset the fight OR their head. Keep them so busy ducking they don't have time to think.