she said that my posture was correct.
You may very well be correct. Or, your teacher has no idea what is correct or not and only knows what she was taught which could be correct or, incorrect.
A lot of what teachers think they know about teaching technique is in error. At least in my opinion. Just because it is what we were taught, does not make it correct.
Ask your teacher how the pronator and supinator muscles are used in playing. Ask them when you should abduct the fingers (never). Ask them what the point of sound is. Ask about arm weight. Ask where the piano keys are lightest. Ask about flexor verses arm weight. Ask what in/out is for. Ask what point of sound is. Ask them when you cross your thumb under the palm why your index finger subtly moves. Ask if you should be crossing the thumb under. Ask if you should ever use your elbow (yes). Ask if the pinky and ring finger are as strong as the others (they are when properly aligned behind the forearm). Ask what ulnar and radial deviation is. Ask how you determine bench height. Ask what a dual muscular pull is. If they can't answer or give a wrong answer, then what your teacher doesn't know can hurt you or hinder your progress. You should be having eureka moments and noticeable progress.
Keep in mind, some really good teachers won't tell you these things during the course of lessons because messing with something that works can make things worse.
A lot of what we are taught has been derived from virtuosos telling us what they feel or from watching them and imitating them. The problem is we look at their hands but the real movement of playing is hidden in the arms through rotations, ups, downs, ins, outs and equal/opposite motions.
I had a teacher tell me to caress or scratch the keys (carrezando) because that is what he felt. It hurt my long flexor tendons to do that. I only learned later that caressing was what he felt from the shape his wrists created as he moved up/down/in/out. I was forcing a caress while he felt one as a result of moving properly. I was still not moving properly despite his instruction. This was because he played correctly but didn't know how or why or what went into creating proper movements. Good effortless technique is a symptom of moving properly. When you do not use the incorrect muscles, playing because effortless. Just one improper movement can cause problems.
Two other things, since gravity is the foundation of depressing a key, you don't need finger strength. In fact, your fingers don't have muscles. They are moved by the muscles in your forearm. With in/out/up/down/forward/gravity/pronation/supination/tapping/not abducting/not equalizing the fingers/playing from the arm . . . you don't need to flex. Rotations using the pronator and supinator are fast, indefatigable and expedites leaps, trills and tremolos.
It can be frustrating finding a good teacher. It took me several before I found one who knew how to undo all my bad habits that 15 years later I am still trying to eradicate. It is best not to hard wire them in the first place. BTW, technique is all in your brain, not your fingers. Others who were trained alternatively will vehemently disagree. We only know what we know and don't know what we don't know, you know?
If you wake up in the morning with stiff fingers, you are doing something wrong. If not, you are quite possibly doing everything correctly whether you know it or not.
