[koreth1]

I had a similar experience learning Mandarin as a native English speaker in my late 30s. I learned to pronounce the ü sound (which doesn't exist in English) by getting feedback and instruction from a teacher about what mouth shape to use. And then I just memorized which words used it. It was maybe a year later before I started to be able to actually hear it as a distinct sound rather than perceiving it as some other vowel.