advice needed for unreasonable 200 m time

I want to improve my 1500 m swim time, in the last few months I swam mostly at aerobic speed. I've recorded 32'41" for 1500 m on July 22 and 3'54" for 200 m on July 20. On Sep 19, I have taken a standard CSS test - 400 m followed by 200 m, and got 8'37" and 4'11" - which was about the same as all my previous tests in the past 2 years, without any improvement. However, after I plug in my numbers into the prediction website: www.arhy.org/swim-predict This set of numbers is unreasonable for my 1500 m speed in July, therefore I redid the test on the next day (Sep 20), with increased effort level, and got 8'1" and 3'54" (my previous record for 400 m was 8'22" - i.e. 21" faster on that day). These set of numbers were reasonable for my 1500 m speed now, but very unreasonable for my 100 m speed - I can easily swim under 1'50" / 100 m one-off with complete recovery between, although not continuously. so my 200 m time was unreasonable. On the next day, I tried 200 m again with further increased effort, and got 3'48". Just before the end of session, I tried one more 100 m at race speed - and got 1'37", further confirming that my 200 m time was unreasonable - I should be somewhere around 3'22" given appropriate training, Therefore I attempted the following in order to find out how can I complete 200 m in the shortest possible time, with increased effort every day: swim the first 100 m like racing 100 m, and force myself through the remaining even though I cannot sustain the effort swim the first 50 m like the first 50 m in 100 m and try to keep that in the 2nd length, but ultimately falls off in the 3rd and 4th length swim at slightly slower pace than 100 m race, and attempt to keep that for all 4 lengths I press my stopwatch at 100 m and at 200 m finish, and got the following times on two different days using various strategies: 1'42" split / 3'41" finish 1'46" split / 3'42" finish I also counted my stroke every length. My minimum SPL / 50 m at warm up is 42 at 1'3" / 50 m, however, in my 200 m tests with all-out effort, under most circumstances, starts at 52 SPL and ends at 58 SPL - a lot more than warm up pace - sometimes even going over 60 when I can't sustain my effort after going fast at the beginning 100 m. I also have another problem - the above times can only be done once per day. After doing such all-out effort 200 m, I feel the pool is SO DAMN HOT and have to get out of water immediately, even after 10 minutes of resting on the pool deck with wind blowing I can still feel fatigue in my arms, shortness in breath and elevated heart rate (more than 100 bpm). No amount of rest and recovery can bring me back to the beginning state in the same day. If I force myself and attempt to do such 200 m again, my time will be at least 3 seconds slower, even with more than 10 minutes of rest and recovery swims. At the end of the session, no matter how slow, I can't get the opening form like that in warm up while in cool down (e.g. I got 42 SPL at warm up but impossible to get below 50 at the end of the session no matter how slow). Is it because of the pool temperature being too hot? I also tried to have some intervals - I tried to swim 55" / 50 m on 1'15" but couldn't do so, falling to 1' at the 5th interval, and the symptoms of fatigue, prolonged elevated heart rate and shortness of breath set in - despite making 3'41" on 200 m. What's the reason about that?