well im not exactly sure what to tell you at this point. im largely a 100m/110mH runner, so ill give you what advice i can. start doing accel dev, intensive tempo, and some general running work. thosell be your 'hard' days, your high intensity days. between those do extensive tempo. so your week might look like:
M: accel dev
T: extensive tempo
W: Intensive tempo
Th: extensive tempo
F: general running work
Sa: more extensive tempo or rest
do this until you get a better plan, and then start over with that (itll probably be a couple weeks at most before you get something better or decide to stick with anything). ask 400stud or DaGovernor for advice there (or really most people on this sit, but they train for the 400) because i dont know if my advice is sound for 400 running.
i also suggest finding some summer meets if you can where you can test your 100 and 200 time. yes your 200 time seems abnormally slow for someone who runs a 51.5 in the 400, so i recomend retesting. your 100 and 200 times will help you get to know which qualities of the 400 you accel in and which you need help in. lowering your 400mH time to sub53 youll need to (1) decrease your 400 time and (2) decrease the defecit between your 400 and your 400H times. so thats all ill post for now, this is kind of long. if you have more questions, ask.
thoughts from anyone else?