The differences between American and British English are relatively minor. When Americans and British people communicate, with a few exceptions, we understand each other readily (though the British are much better at understanding Americans). I'm not sure why you would think that speaking with one form of English as opposed to another would somehow pose a problem. English is becoming less American and British, and more international everyday. I'm an American, but commonly use words such as cheers, amongst, and whilst. In turn, I have heard people from the UK use words such as cool and shopping cart. So really, if I were you, I wouldn't sweat which form of English you speak or what kind of accent you may have, because as mentioned above, the reality is that in all likelihood, your accent is actually more a reflection of your mother tongue and will be recognized as such by native speakers of English.

With a few exceptions, the main difference between American and British English are the accents. You most likely speak don't speak with either a American or British accent, but a Brazilian one. If you're worried about being understood then talk to Americans on Skype to see what they think.

You mean you speak American enghish with a British accent? I do not think it matters a lot. Maybe it will lead to some misunderstanding. But there should be no obstacle when communicating with other people.