Find videos on Youtube which talk about where your tongue should be in your mouth to make certain sounds and how native speakers say certain sounds. I had a friend, from Spain, who couldn't distinguish the English 'y' and 'j' because she didn't know how to make the sounds properly. Little by little she got to grips with it, once she got accustomed to getting her mouth in a certain position. If you can, get feedback from native speakers. Unless they are teachers they probably can't articulate what their mouth is doing when they say a certain word, but maybe they can show you instead.Get to grips with the schwa sound. Oh, and listen a lot. Like really listen to how native speakers say words, especially the vowel sounds.