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.
Students: Are you brave enough to let our tutors analyse your pronunciation?
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.