My biggest mistake in language learning (2/2)

If one were to achieve upper-intermediate or even higher proficiency level without knowing the correct way of spelling the unique sounds in one's target language, it would be excruciatingly effort-and-time-demanding to reestablish the speaking habits. Without deliberate pronunciation training it is almost impossible to avoid such mistakes. Although I learned a lot from immersion, it has not taught me how to pronounce and, most importantly, distinguish different sounds. If I were to give a piece of advice to somebody who did not deliberately study phonetics (except for the first few lessons) at all, I would highly recommend starting right now. Not everyone, including me, needs to have a native-like accent. However, I find it crucial, especially for someone who aims to be an advanced speaker, to master pronunciation so that it would be impossible to guess one's origin. I see it as a turning point because if one can pronounce all the sounds correctly, without substituting them with the sounds from one's native language, one has mastered the former sounds to a native-like level. However, it would take hundreds of hours of mimicking (through shadowing, for instance) to have an undistinguishable accent which, again, is not for everyone.

Hello! I am aiming for a formal style of writing. Feel free to correct not only grammar mistakes, but also things that may sound unnatural. Thanks for your help!