Never say never, I guess.  But I would say the South almost never had a chance to win that war.  They were so outnumbered, outproduced, outgunned and outmaneuvered there was little they could do.  It is a testament to their military leadership and their determination, actually, that they lasted as long as they did.  Only British intervention would have given them a legitimate shot at independence.

I don't think you can say the South "never" had a chance to defeat the North. However the odds were definitely against them. The North had the infrastructure to fight a longer war than the more agricultural south. If the South coud have formed an alliance with Britain it would have made a difference. Why were they not able to form that alliance? The South had most of the goods that other countries were in the market for.

I'll reduce the argument to several simple military "what ifs." 

(1)  What if Robert E. Lee's "lost order" had not fallen into Union hands prior to the Battle of Antietam? A decisive Southern victory would have been probable, and Washington D.C. would likely have been captured. The Union army would have been hard-pressed to stop the Confederates without this important knowledge. 

(2)  What if James Longstreet had convinced Lee to move around the Union army at Gettysburg and set up a defensive position in a different location?  The bloodbath at Gettysburg would have been averted, and Lee's army would have lived to fight another day on Northern soil. An additional "what if" could include J.E.B. Stuart's failure to provide cavalry support and info concerning Union troop positions. Lee's army would not have been surprised at a place he had no desire to defend. A third Gettysburg "what if" could include Dick Ewell's failure to take Cemetery Hill on the first day of action. It was a no-brainer that it could have been captured with a small number of troops; fortified, it would have stopped the Union army cold. A major victory, at Gettysburg or elsewhere in Pennsylvania, would have meant a probable assault on D.C. by an unstoppable army.

The Confederacy could have won, barring foreign help, if they had been able to take the war North to the point where it was politically unfeasible to keep fighting.  Had an alliance with Britain been established, the North would have been fighting a two front war between the Confederacy and Canada, and would most likely have withdrawn.  But North on South, with no outside resources, doomed the Confederacy.  The North had almost all the industry and railways, and a preponderance of the population; these factors alone made them unbeatable, despite the South having the better military leadership.

In addition to the ideas above, another factor was economics. As I understand it, the North was far more industrialized than the South. As pointed out in #2 above, the population was also far greater, leading to a bigger workforce to support the much larger numbers of factories in the North. They could financially support a war in ways that the South couldn't.

Pohnpei 397 did a great job of setting up the main points for you. Understanding the motive behind thewar what the north’s real agenda really was and what the south’s real agenda was will go a long way toward helping you to decide exactly how your thesis should be worded and what sources you will need to use. For the record Pohnpei 397 had a well written thesis too.