The Battle decimated the troops, but it also affected their morale and their own belief they could win the war. It was a decisive battle for both sides, and when the South lost, it was another element that would lead to their defeat. Union victories in the West (Vicksburg and Port Hudson) simply added to the South's woes as they retreated from Gettysburg. Finally, the third important factor in the South's defeat was President Lincoln's reelection. The Democrats made the war a major part of their platform, especially the freeing of the slaves in the Emancipation Proclamation, and their candidate, had he won, might have recalled the petition and changed the tide of the war. However, Lincoln's reelection showed a majority of the country supported the president and his policies, and that included his staunch position on the war. The Democrats wanted peace at any cost, but their nominee (General McClellan) proposed both peace and war. He did not support emancipation for the blacks, but he did support suppressing the South by force, and so, he did not gain enough Democratic support to win the election....

Lincoln's reelection tied the North together and showed the supported the war effort and freedom for blacks. It sent a strong message to the South - that the North would not back down in this war and ultimately, the South could not overcome these three important issues. Each one was vital to the ultimate Southern defeat, and without any one of these elements, the tide of the war could have ebbed and flowed on a different course.
The South made mistakes during the Civil War, and so did the North. However, without these vital turning points, even with other obstacles, the South had at least a chance of victory. With them, the South was doomed to eventual defeat, reconstruction, and reuniting with the North to form a new and different Union.

Reference

McPhearson, J.M. (2001). Ordeal by fire: The Civil War and reconstruction. New York: McGraw Hill.