I have question concerning the zikhr in namaz.

I noticed that before, I made many mistakes in the makhraj in namaz.
So, I learned the correctly pronounciation of it.

But, I still have difficulties to memorise it.
So, during my namaz, I repeat 3-4 times the same part of sentence to make the correct makhraj.

My namaz is still valid right? Even if I corrected it many times?

Your namaz is not invalid for correcting mistakes in pronunciation. However, try to spend time learning to avoid correcting it during Namaz.