Well I'll assume that there are probably dozens of answers and responses to this question albeit asked in different ways so that's the only information/criticism I can give. However I'll give as forthright and concise a response as possible and take it in two parts.

1. How close did the Confederates come during the Civil War

The answer is not very. This is somewhat of a simplistic response and certainly the Confederacy had its moments were they seemed just on the precipice, but ultimately the closest they probably came was during the campaigns by Bragg and Lee during the early Autumn of 1862. Losses at Perryville and Antietam robbed the Confederacy of their best and last chance to gain international recognition and support. So to answer your question they came close to a potentiality that may have potentially turned the tide of the war, thus my response of not close.

2. Why did the Confederacy lose

A far more intriguing question. The easy way out would be to say that the North simply overwhelmed them with more men and material than the Confederacy could match, but that would be a grave injustice to the Union Army and would allow a multitude of Confederate errors and blunders to escape intact. Firstly, the Confederacy had a significant problem with the lack of overall strategic vision particularly in the West. This lack of vision mainly lies at the heart of another blunder and blunderer Jefferson Davis. His unwillingness to commit to an overall commander for all Confederate forces and even more egregiously a single commander for the West, at least after Sidney Johnston, robbed the Confederacy of the unity and singular vision it desperately needed. The Confederacy's desperation to be as decentralized as possible crippled its military efforts and hampered its wartime machine, this lack of an overarching unity or single minded purpose is a recurring theme and an important one. Davis propensity for playing favorites and micromanaging his strategic and tactical betters was another difficulty to overcome. The lack of overall unity amongst the various Confederate states is staggering and as the war dragged on more and more governors looked no further than their borders. Desertion was a huge problem on both sides, but the Confederates had a much harder time policing it than the Union because such things required the cooperation of state governments, which as already stated were rarely cooperative. I could go on, but I'll desist and give you the opportunity to add further questions, ask for clarifications, and hopefully enjoy this post.