I have a problem that often get on my nerve: I feel I have blended the British English and American English in my pronunciation. I wonder if I must make a choice between the two and then get rid of the other from my pronunciation. It sounds necessary, but gush... how? Do I have to look through the dictionary from A to Z? I'm afraid I cannot do that because i don't have the time. So could someone help me out?

Any comments are highly appreciated.

No, not at all! My pronunciation is a mishmash of American and Canadian and Japanese-English, plus a bit of awful Brit accent I picked up on the stage. Everyone's pronunciation is a little different-- it is a part of your character; so unless you are a movie actor, relish it.
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Accent doesn't really matter. To be understood is the main thing. Accents are cute!
Don't worry too much Ding, "International English" - ie that spoken by many non English speakers - quite often is a kind of "mid Atlantic" mix of American and British accents and no-one would find it odd. Like Brian says, as long as the meaning is clear, that's what matters.