Perhaps, one big and common obstacle for you to improving your English pronunciation is the influence of KATAKANA. As you know full well, the sounds of borrowed English words are represented in Katakana whose vowel sounds are limited to five (5) whereas in English there are no less than 18 vowel sounds, excluding those consonant sounds which, sometimes, act like vowels sounds like y and w. In addition, there are sounds in the English language which do not exist in your mother tongue such as the /æ/,/ʤ/,/θ/,/ð/,/l/,/r/,/v/.
Comparatively, the way you use the speech organs to speak Japanese, as well as the speech organs themselves, are, to a great extent, dissimilar. The stark difference is the use of vocal cords, the jaws and the diaphragm. When you speak your mother tongue, you hardly use your jaws, your vocal cords and diaphragm, but when you speak English you need to use them all the time. There are English sounds that requires jaw drop and opening of the mouth such as the so-called “American a”; and if you don’t do this, you’ll end up making a different sound.
That’s why, in order for you to learn English pronunciation more effectively, you need to know the factors that hinder your progress and know how