The timing of the lowering action is a thorn in the side of many-a-novice. The difficulty we have here is that you could actually be lowering too early OR too late, and without actually seeing you to know which it is, we could very well be giving you the wrong advice. From the way you describe as your teacher's explanation, it sounds as if he or she is saying that you lower too early or too quickly. And yet, in my experience as a teacher, I have found the opposite to be true more often. The best I can do is discuss lowering action in general, and hope that you find something helpful in the discussion.
Traditional English examinatioin technique (a la ISTD) tells us that we must maintain a certain amount of pressure in the foot on which we are landing, so that the heel does not lower completely until the weight has fully arrived over it. Now that's not to say that the heel should remain as high as possible off the floor until the body arrives, and then lower all at once. What it means is that you should lower the heel gradually as you move toward the foot, so that you and the heel arrive approximately at the same time.
More modern standard technicians have relized that, while not a bad training tool for beginners (especially those with weak feet), so much foot pressure ultimately tends to be more detramental to the movement than it is beneficial, particularly with respect to forward steps on count 3 (back steps are a slightly different story). It is now thought (especially amoung competitor types) that one should "lower behind the foot", meaning that the heel will lower before the body weight fully arrives over it, with very little to no resistance from ankle pressure. You want to have fully lowered by the time your feet come together, including bending your knees to the full depth of the down-swing.
Regardless of which interpretation you choose, it is important to understand the common thread: On forward steps, the body level should move down as it moves forward through space, arriving in a fully lowered position as the feet are passing. If someone says you are "falling", that would imply that you either lowered straight down for a period of time, causing an awkward break in the forward momentum (too "early"), or that you didn't lower deep enough or soon enough, causing you to fall into the next forward step. The question for you, of course, is which of these applies to you.
Those who don't lower completely by the end of 3 and "connect their weight" firmly into the foot will tend to fall or lurch forward slightly out of the foot. This results in a weak forward drive on the next 1-count, and quite frequently, additional lowering onto 1. This is by far the more common mistake.
Lowering too early -- a far less prevalent problem with beginners, IMO, but definitely possible -- is caused by lowering too much all at once, whereby the body actually stops moving forward for a moment. To fix the problem, you must make sure to continue drawing your weight forward as you lower. This is especially true of the low center and back leg, which will tend to remain still as the high center and upper body continue forward, causing you to pitch.
To be sure you are doing it correctly, begin with your feet together, high on toes. Slide your left toe along the floor for as long as possible. Your body should move forward, too, to stay about halfway between the feet. And as a beginner, you have to trust me on this: Slide a few inches farther than you are naturally inclined to. Go just outside of your comfort zone. Get a good, solid division in your legs. At this point, you will have already lowered somewhat by virtue of the leg division itself, even without having bent the knees or having lowered the heels at all.
Once you hit the "mid-stride" position, this is where the knee bending and heel lowering begin. But remember, it only begins here. You shouldn't be all the way down until the feet actually come together again. Try to balance the bending of the knees, lowering of the heels, and moving of the body forward so that all 3 actions are complete at exactly the same time (although if the heels come down before that, most non-ISTD book types agree that it's ok).
Also, equally important is that you never allow your upper body to move forward without the lower body. This is the inevitable result of moving the body while the feet remain in place. You must never, for even a split millisecond, allow both feet to remain stationary. The very second that left foot ceases to move, you must immediately begin moving the right foot from behind, not allowing one inch of body movement forward (or downward) in the interim. If you do, you will necessarily cause yourself to pitch forward slightly, contributing to the feeling of "falling" onto the follwing 1 count.
When you do arrive completely over the left foot, you should not have your right foot pointed very far behind, if at all. If you do, it's a good indication that you did allow a significant amount of body movement forward with stationary feet.
I hopethis helps. Good luck with your Twinkles!
Regards,
Jonathan Atkinson