Our native language influences not only how we sound when we speak a foreign language, but also what we perceive when listening. When listening to English, you hear the sounds through the filter of your native language's sound system. This means it can be difficult to notice your pronunciation differences, because to you, it sounds the same as what you hear when Americans speak.
The good news is, we can overcome these filters and re-train our listening and speaking to include new sounds, and to differentiate between similar sounds. Systematically exposing ourselves to the American English sounds in combination with imitating them can help us learn to produce them correctly. You can read research into this phenomenon here.