bcz after world war 1 and 2 america's power increases and US kept Britain down,US is the biggest reason that Britain never increases its power and even went down, now Britain is just a puppet of US, British people don't like much US,

Adding to Jura, due to the declining resources, they could no longer hold our country as their colony and we gained our independence on 15th August 1947 :D So did many others gain their freedom. Since Britain was exploiting all these colonies to make money, this added to the decline of their power.

World War I was a huge drain to Britain's resources. The Great Depression weakened it still further. Britain might have recovered eventually however, if it had not been for World War II. Unappreciated by many Americans is the fact the Britain sold all of its extensive overseas financial interests to pay for the war. It was the final, crippling blow to the British Empire. Britain would have eventually lost its colonies anyway. However, it would have still maintained a much greater degree of power and influence had World War II not happened. This is grossly simplified, of course, but those are the main reasons.

All powers decline. Nothing lasts forever. Don't they teach that in China?

The 2 main reasons were terrible loss of lives and the huge drain on British economy.