Is there any of you who used to speak American then finding out that you are eventually changing from American to British accent from time to time just by listening or watching any British-made programmes?

That's what happening to me...
Yes, this almost always happens to me when I watch British programs (I am Canadian). Interesting to know that I am not the only one. I don't find I pick up any specific British accent, but I find that I pronounce different words in different accents.
The British accent sounds better in my ears but oddly the words come out easily during any conversation if I put on the American one.....
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I find myself doing this sometimes, especially after I have watch British shows. I have been told by my Dad and brother that I was talking like a British person and to talk normal!

I am from the U.S.
I tried, but failed.

It's really difficult even though English is not my first language.

I love how British English sounds, but I feel more comfortable and confident when I speak in American English.

Besides, some vowels, like British 'short o' sound is really hard for me to pronounce.

It's probably because I learned American English first.
I didn't necissarily switch accents...

But I spent about two years in London for work and in about the first three weeks I started having difficulty pronouncing words. I'd trip over my tongue often and started chopping off syllables. It ended after a while but for some time I had to consciously focus on enunciation to get anything out.

When I came back home for Christmas to my family my mother noted that I had dropped the harsh 'a' sound and was saying 'ah' instead. But other than that I picked up my old Washingtonian accent immediately back in America.

My accent was definitely effected by exposure to British accents but it was more of a watering down than anything else. I'll never get a pure British accent (and I'm not trying to) and British people can still always tell that I'm an American.

I don't know whether or not you're talking about native or non-native English speakers though. Because you can learn BritE and learn AmE and transfer between the two as well. It just depends I suppose.
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Yes. And it happens in every language I know. When you're concentrating on a language it emplace in your unconscious. I am Iranian and I'm learning my 5th, I experienced that in all of them.