It's time to talk about balance fundamentals. One of the reasons why Taijiquan is practiced slowly is so that you have time to feel your balance. To describe balance, Tai Chi teachers use the scale of yin and yang, with the emphasis on yang (substantial).
So with reference to your feet, if you are standing, and your right foot is 0% yang, then no weight is on it. If your left foot is 100% yang, then all your weight is on it. Got it?
When doing the form, beginners and even some intermediate students often have problems stepping. Their feet don't come down gently on the floor. They tend to move too quickly onto their new foot. They often wobble while they are picking up the trailing foot. The reason why this happens is because they are not correctly balanced.
When people walk normally, they are almost permanently off balance. Walking is about falling over, but stopping yourself by stepping forward. Tai Chi aims to improve the balance by reducing the need to overbalance when stepping.
Although correct posture plays a part in balance, the fundamental reason why people don't step comfortably is that they are not distributing the weight between the feet in a correct manner.
The solution is simple. We are going to walk two steps. If you are stepping out with your left foot, make sure that your right foot is 100% yang before you move. You can then place your left foot exactly where it is meant to go. Place the heel down first, then the toe. Once your left foot is appropriately placed, you can slowly shift your weight onto it until the left foot is 100% yang. Here is the important bit. DO NOT RAISE THE RIGHT FOOT UNTIL THE LEFT FOOT IS 100% YANG. If you do, you will most certainly overbalance.
How do you know that the left foot is 100% yang? Because you will not be able to feel any weight on the right foot (0% yang). Now the right foot is raised, pause a little bit to check that your balance is good, then step out with the right foot. Now transfer the weight to the right foot. Remember, don't raise the left foot until you feel that there is no weight left on it.
Repeat a few times until you get the hang of it......
Now do it again without looking at your feet.
If your posture is poor, your balance may also be affected. As well as Tai Chi, you could also try Alexander Technique exercises to supplement your training.