You have to remember that a cessna 172 is a nose heavy aircraft when its flying with slow airspeed. I fly a cessna 150, I usually keep 60 KIAS on final, and when I am about 25 feet above the runway, I just pull the throttle to idle, and just glide in. Are you forgetting to flare? I hope it helps. If you have PAPI lights use them for your descent.