As a student, of learning any language, the tongue, jaw and lips all have to be in just the right position in order for the correct sound to come out --- and for people to understand you.
One aspect of English, is that the mouth needs to be more open. Have you looked in the mirror to watch your mouth and see that you are opening it more? Everything is in the right position? If you can't quite get the correct pronunciation, then "something" is not in the right position. If teachers, or native speakers of that language are asking you to repeat, then it's time to practice and ask a teacher. All teachers are supportive and want you to do your best. So we can understand you too!:-)
Would love to hear your thoughts or suggestions, on how you practice pronunciation.