There are many sensei who believe that they are well versed in the application of technique and they can show this by taking each one to its fullest limit. That would be fine if the human body never incurred injury, or had a bell that rang when it reached its limits?
I understand the over extension and application of some sensei, because they have never gotten a feeling for the human body and its response when the muscles and joints begin to reach their limits. Been there, gotten injured ... nothing but pity for your plight.
On the other hand, are you learning how to bend with the wind?
There will come a time when the situation is for real, and the pain/torture of that practice will be welcome. At some place, some time, some where ... you will encounter a violent situation. If that pain you encounter during practice is it, then thank what ever gods you curse or worship.
On the other hand again, if you have to look inside of yourself to reach that point of saying NO! ... then it had better come out of your mouth, excuse yourself, and go sit down.
It is not childish to reach your limit, know you must stop, and execute your own will. If the practice or participation of practice is physically harmful, speak up! Sensei can't read your mind?
Hopefully, as these teachers grow older they will turn pain into gentleness.
The best practice I have ever had is with older sensei's whose techniques were like a whisper touch, as fast as the eye could percieve, and flowing from beginning to end!
Why? Because, maybe, they had the same lessons you are getting now. What are you going to do with the lessons you learn, and the problems you face?
Keep talking about it, with your sensei, your friends, and sometimes strangers ... you can never tell where the answer will be found?