This is why special endurance reps are important. You need to learn what it feels like to go out in your goal time. So in order to do this, in training you run your special endurance reps slightly faster than you need to go out in the 400. So if your PR in the 200 is 22.0 and you want to go out in say 23.2 and come back in 26.0 then during special endurance reps of 200 meters you should be focusing on running under 23.2. Sme thing for the 300 special end reps. Going out and running them in around your goal time will teach you what it feels like. And the last 100 of a 400 meter race will always be hard. As Alex said you just need to hang in there, stay relaxed, and make a consious effort to increase your stride frequency when tying up.