The 172 can be tricky to land if you come in too fast. It may be worth asking your instructor to demonstrate some short-field approaches, with an approach speed lower than the one you are using normally.
You could also try a smaller flap setting. Try these different techniques at a safe altitude initially to get accustomed to the feel of the controls and the attitude and power combination.
If the time from flare to touch-down is more than 2 or 3 seconds, and the touch-down is at the correct attitude, your approach speed is probably too high.
Everyone goes through phases of having trouble with landings. Don’t worry too much about getting a smooth landing but concentrate on a good, stable approach and correct flare technique. As others have mentioned, looking at the far end of the runway during the flare may help too.
Good luck!