In order to do something, first you have to learn what that something is. If it is a new activity, it can take a while to figure out how to do it correctly, as even if you mentally understand what is being asked, you may still be far away from executing it well.
All activities start out like that. At first, you don’t throw or hit the ball very far, you don’t catch it much of the time, and you get tired or clumsy while trying to do all of that. Later, you get the idea and get more coordinated, and you can more or less do the basic thing you have been trying to learn. If, however, you expect to continue to improve, you reach a point where someone with an experienced eye (or ear, or both) has to intervene, so that the refinements which lead to excellence can be made. The sooner such intervention takes place, the better it is in the long run, if you have high expectations, because it is harder to correct a bad habit after it has been in place for a while than when it is new. The more complicated the activity is, the longer it can take to learn it, the more factors are involved that have to be done correctly and at a high level of ability and the more small differences can “make it or break it” at a professional level, and the more all of this matters to your overall stamina.
You can finally arrive at a point where you know what you want to do, you know you can do it, you know you are doing it fairly well. You are strong enough (and flexible enough) to do what you were seeking. You may not, however, have the stamina to keep doing it for a long time. Stamina is what happens when you can sustain doing your activity at a high level over and over and over without undue fatigue. That can take years, maybe even decades. It is very hard to run a marathon, to dance the lead in a major ballet, to play a concerto or to be in the Olympics (although not in the same way), unless you have developed not only skill but stamina.
From the standpoint of vocal work, the things that take the most stamina are leading roles in an opera sung with a full orchestra in a big house, a leading role in a big music theater piece, done eight times a week on Broadway or on tour, and singing in a rock band on a tour. Speaking in a play would be the same, too, if the role were long and powerful, and the run was long or a tour.
So, if you are a young person, and you are just beginning to learn to sing, and you are being asked to sing the lead in a rock musical at school and you are also being asked to sing classical songs in your lesson and you sing in church in the choir, and you like to sing at home with the radio, you are going to find your voice very tired, no matter how well you sing and no matter how good your lessons are and your technique may be. Young throats just do not stand up to constant use unless you are a VERY unusual young person. You are much more likely to encounter both vocal health and musical issues than someone else who is doing EXACTLY THE SAME THINGS but is 10 years older than you are, or 20. There is a reason why the profession has been, for 200 years, conservative about young voices.
It doesn’t mean you shouldn’t do the various things mentioned above, it just means that you need to approach doing them carefully, slowly and with correct supervision. You must understand that STAMINA is something that comes only in good time. If your teacher doesn’t understand that, please inform her.