I suggest you change you training schedule and have one day a week that you do 200m intervals. For these you set your watch for 45 minutes and do as many 200 intervals with a 1 minute break beteen each one. This should help you speed in the 400m and it will pay of in the end.
Doing a long run a day will help. Do some 800m intervals at first a couple of days a week, and then a couple of weeks after, start moving down to shorter distance intervals. plenty of carbs a couple of days before the race too!
You must practice long distance run so that your metabolism sets up to the higher range and u dont get tired faster.Drink less water before running to avoid belly pain.
Practice makes perfect. You need to increase your stamina, try running up hill, in the sand, with a weighted vest, breathing exercises could help as well.
