Hello Miss Hello,
I'm surprised nobody else has offered any tips yet. So many people have written that pronunciation is important, but they seem to have no idea how to actually improve it.
From your posts it is obvious you know a lot of English. At this point in your study, you not only have the problem of learning English pronunciation, you also have the problem of breaking bad habits formed after mispronouncing English for such a long time. It will not be as easy for you now as it would have been when you first started learning English. Here are some things I can suggest:
1) Listen to all kinds of English very CAREFULLY. Today's dictionaries with voice recordings are not too bad, but single words won't help you very much. You have to listen to a variety of voices and a variety of word combinations, as well as sentences and paragraphs.
2) Repeat what you listen to over and over again. Try to imitate native English pronunciation as closely as possible. Native English speaking TV and radio announcers do this, so don't think it's too basic for your English level. In my classes, I have students pretend that they are on the radio, saying sentences just like the professionals. It's not only effective, it's also quite fun!
3) Try recording your voice while reading whole paragraphs, then compare with the native English speakers on CD or tape. Be sure to do this once today so that you can see your progress over time.
4) Try to identify your weak points - which vowels and consonants are hardest for you to pronounce? Put extra emphasis on those sounds. You may also have trouble with groups of consonants or vowel combinations. English rhythm is sometimes a problem for students, so pay attention the this, too, as you listen.
5) Since you're trying to break old habits, you are going to have to FOCUS. Practice several times a day every single day for as long as you need. I think you can see some improvement in a few weeks, though it will probably take longer to reach the level you really want.
Good luck and don't stop trying!