What could be the problem with my 400m?

You have to improve your 100m PR and your 60m which requires doing some speed work. 4x 12 secs is 48 secs. How do you plan on breaking 50 secs if your 100m is only 12 secs? You would need to run your 100m PR +0.5 four times consecutively. That's just not possible.

You have to do speed work to improve your 100m time. That means 30m starts, 60m intervals etc. If you feel lactate acid build up when training you are not doing the type of workout that you need to be doing at this point. You've probably already capped or are close to capping your lactate tolerance relative to your current speed.

You might be thinking: but if I stop training my endurance won't that get worse? Yes, and so what. Once you improve your 100m you can begin doing speed endurance work again and you'll be significantly faster than before. But you have to improve your 100m at this point.

Unfortunately, your HS coach is probably a charlatan and doesn't understand any of this. I wouldn't trust him with how you train but finding out how to negotiate with him so that you can be doing the proper workouts to improve might be a pain.

Faster 60m = faster 100m = more speed reserve = better 400m.

And yeah your speed endurance probably did get worse from last year to this year but that's not your biggest problem right now/the most effective way to improve your 400m long term. You have to get faster. Remember the 400m is a sprint event not a middle distance race.

Yep with those 100/200 times the best you could probably hope for is a 52.x going out in a 25.5 for the first 200 and coming back in about 26.5 for the second 200. Since you're not running that fast yet this year you're probably not going out fast enough or you're dying too much the second half of the race. If you want to run a faster 400 than you did last year you need more speed, if you could get your 200 time down in the mid to low 23s you would have some hope of running 50.x as then you'd be able to go out the first 200 of your 400 in 24.5 or so and then try to come back in 25-26 seconds for the second 200, see how that works? Going out in 24.5 seconds for the first 200 when your 200 PR is only 24 seconds would be suicide and that's the kind of pace it takes to run in the 50.x - 51.x range.

The pacing in the 400 is pretty specific, you want to run the first 200 about 1.5 seconds slower than your current 200 PR, any faster and you will die too much at the end and any slower and you just won't be able to make up the lost time at the end of the race. You don't need a lot of endurance to run a great 400 but that simple pacing is key to finishing the race fairly strong. I've had kids break 50 seconds with no real 400 meter training, but they had 200 meter PRs in the 22 second range and could easily go through the first 200 in 23.5 or so.