I agree with all of flycat's advice, especially the bit about talking to your instructor.
Also, have a read of the Private Flying forum (where this thread will no doubt get moved to shortly anyway) because this subject (in general, if not specific to C172) gets discussed regularly. In particular, have a look for posts by Chuck Ellsworth (sp?), who I think posts as Cat Driver (or something like that... I'll come back and edit this post if I find his name and discover I've got it very wrong!)
Personally, I would guess that you're not focusing your vision correctly during the round-out and subsequent flare. Initially, your attention should be focussed on your "aiming point", usually the numbers. As you begin to round out, shift your vision forward, to a point a few hundred metres in front of you (the exact distance depends on your speed, but for most purposes, focusing on the end of the runway is a pretty good approximation). This works for any type, and for some very complex mathematical reasons that I never understood, it gives your brain the image it needs to be able to judge your height correctly, and adjust your attitude continually as necessary.
Another suggestion would be to get a tail-dragger checkout. This is absolutely guaranteed to improve your landings, since you can not land a tail-dragger safely if your technique is incorrect. A night qualification (if you haven't got one already) will have a similar effect, although it's not quite so good.
Good luck!
FFF
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