If the Colonies could defeat Britain, the most powerful nation in the world, why couldn't the Confederate States do the same?
Some leaders, such as Prime Minister Viscount Palmerston of Britain, believed that the Union could not bring the Confederate States back...and he was almost right!
After Fort Sumter, Lincoln blockaded all the Southern ports. England considered the blockade in itself to represent two warring factions, and declared neutrality in the conflict.
So, the main reason that the Colonies were able to defeat Britain--the threat from other powerful European nations--very quickly was eliminated as a factor.
The English did threaten the United States by building military forces in Canada at one time. This was in response to a Union ship stopping a British ship and taking as prisoner two Confederate envoys on board. It took about five months, but the envoys were returned eventually.
Ultimately, the United States just had a larger army. For a long time, that didn't give them the edge they needed. But time in a war is not on the side of a small population, becaue there are no replacements.
There are two reasons why the Confederate Army held out as long as it did. The South had better above average military leadership, and the North had below average military leadership. But the South's better leadership only was enough to allow them to continue for as long as they did. The North meanwhile made very serious mistakes, which was a large factor in the bloodiness of the war.