The two accents are indeed different, but are mutually understandable, although we Americans do sometimes marvel that the British consider themselves native speakers of English!
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try to pronounce each sound by trying to move forward towards the front of your mouth while speaking. Try to pronounce the word "four" with the normal position of your tongue, and then say it while you are pushing, or gliding your tongue forward and activating your lips area
I quite agree that it's easier for a foreiner to understand an American accent.