What is an elephant? When you see enough animals that people called elephant, you'll form an idea of what is an elephant.
What is physics? After you studied it for a few years in high school you know what is all about physics.
And it is the same with Tai-chi. But unfortunately some tai-chi practitioners after studying the stuff for some years still asking this question. Why?
I dare say after seeing one or two elephants, a person might not be able to recognize one next time, and in particular if the person is not a very observant person. And similar for physics, well, not all students passed their physics exam, and there are indeed lousy teachers.
So giving your sifu the benefit of doubt, you should question your effort putting into learning (and for this matter, you should of course put the benefit of doubt the other way too - and consider finding a new teacher!)
There are many approaches to teach Tai-chi, and even the same teacher may use different approaches for different students with different motivations and willingness to put in efforts. If you're the unlucky one who still don't know what is Tai-chi after a few years of practice, do honestly answer the following questions.
1. Have you practiced zhan zhuang (embracing a tree/standing like a tree/standing meditation) or seated meditation (of Taoist chi-type) during your years of Tai-chi form practice?
If no, you should seriously consider doing some. It is not compulsory in the beginning, but somewhere down your learning path, it is an essential supplementary practice - for you to get a much needed understanding and experience of chi.
2. Do you feel it better half-closing your eyes when you practice your form and your mind won't wander away during practice?
If not, you should first cultivate your chi through learning standing or seated meditation.
3. Do you feel a resistance inside your body when you do the form slowly and you kind of enjoy the internal resistance?
If not, try doing some zhan zhuang or seated meditation before your form practice. This is a procedural recommendation, i.e. I have assumed that you have learned zhan zhuang or seated meditation.
4. Do you feel specific resistance in your ball-and-socket joints (shoulder joints and pelvic joints) during form practice?
If no just to this, don't worry, you'll get it in time. You are now ready to tackle the 10 Essentials of Tai-chi mentioned previously in this blog, and good luck with your practice.
5. Do you feel sharp pain at your joints during form practice, in particular at your knee joints?
If yes, ask your teacher for advice as how to avoid them during practice. If your teacher has no solution, find another one!