According to my experience in teaching Arabic to many people around the world, I can tell you what are these Arabic letters confusing you :
1. 'ع" Do you think you pronounce "ع" properly! maybe you are saying it "أ"
2. "ح" this is the stronger sister of هـ So make you sure you're saying ح not its sister
3. "ض" Maybe you think that ض is easy for you..
if there is another Arabic letter confusing you, I'd like to know it.