there are two reasons for this. as two others have pointed out, pacing is important. if you go out too fast at the mile you tend to die hard, particularly over the third lap. the other cause is endurance. you simply don't have enough miles in your legs to be strong enough over the mile to do your speed justice.
the solution is therefore twofold.
1. run more. you don't have to do anything clever or fast, just increase your mileage gradually and you will get better.
2. try going out at a more modest pace for the first lap. when you are older and stronger you should be looking at around 5 seconds per lap difference between 800m and the mile. for now, think more in terms of 6 - 7 seconds so your 800m time is 60s per lap and you should be going out at around 66 - 67s per lap for the mile. try that.
good luck.