How do i swim 100 fly without getting tired keeping long stokes and a good technique?

im a freshman and on the varsity swim team our 1st meet is in 3 days. when my coach says to swim stroke i swim erfly every time. i dont change it up at all so on count of that i think shes going to put me in the 50 fly (yay) and the 100 fly. i swam 100 fly maybe 2 yrs ago and i thought i was going to die/puke any second when i was done. how do i swim fast, not get super tired, keep technique/ rhythm, not get sick and build my confidence to believe i can do this no problem... *some extra info* ive been swimming for 9 or 10 years now 4 of those years on 3 different swim teams the rest of the years on 2 different swim teams