READ OUT LOUD.... en voz alta, slowly and loudly. This gives your mouth more practice in making the sounds. But practice only what you know sounds correct. Take sentences you want to say easily. Say them to yourself loudly many times. This will help mucho.

I agree with Alan, you must speak with native speakers (especially those who do not know your native language and could not help you). It will be difficult and slow at first, but you will get better with time!

Talk slower.

Thanks a lot! Now I have to find someone who can help me :)

Listen to and talk with native speakers. That is the best way, in my opinion. Because you can get instant feedback. (Of course, pronunciation varies by region.) Besides that, get as much native-speaker input as possible: surround yourself with, immerse yourself in English-language music, movies, podcasts, radio, etc. Even if you do not understand everything, you will be exposed to common words, word patterns, sentence rhythm, vowel reduction...