Hi Luisa, 

I'm so disappointed to hear about your experience! As long as you have some basic command of the language in which you will be conducting business I think it is absolutely abhorrent that you could be passed over for a job because of your accent. You obviously can communicate well in English, both given your well written question and your ability to go through the whole interview in English. The fact that this company told you an 'American' accent is a requirement is utter BS. There are dozens of accents within our huge country, so which one does he want you to have?? I want you to know that the vast majority of the Americans I know would find this ridiculous, and I do not think that your accent is likely to prevent you from getting other jobs... and if so, I agree with the other commenter who said you're probably better off not working for that company anyway. 

Anyway, I apologize for the little rant. I'm sorry I don't have any good advice for your first question. For your second though, I personally feel proud and grateful when I hear people speaking English with an accent, because it reminds me that my country was (and still is) defined by immigrants, and that regardless of the craziness of the political climate, it is still a country that people around the world dream of coming to. I also feel a some amount of cultural embarrassment, because many people who were born here have very little interest in learning other languages, thus making it really important that immigrants learn at least a little English. In other parts of the world knowing multiple languages is commonplace, and I think it's ironic and a little ridiculous that a country built by immigrants is so resistant to accepting other languages. 

Best of luck with your job search, and I hope that you'll retain at least a little of your accent!