Instead of continuing the campaign, where he had an advantage, McClellan demanded reinforcements, and the campaign missed a golden opportunity to take the capital. McClellan blamed the mishap on the inability of Union troops to join him on the peninsula to aid in the attack, because they were engaged in the Battle of the Shenandoah Valley in the west, but many doubt this, feeling McClellan was simply afraid to attack. In the Battle of Seven Pines, McClellan split his army into two positions on either side of the Chickahominy River. General Johnston's attack could have wiped out at least half the Union forces on one side of the river, but the attack was complicated and confusing, poorly executed, and the Union forces repelled the Rebels. The battle also brought about the command of Robert E. Lee, who replaced Johnston who was wounded during the battle. This would give the South a strategic advantage, as Lee turned out to be an excellent general. The Seven Days Battles were launched against McClellan's forces still on the...

In fact, McClellan ended up retreated when he could have counterattacked.
The Second Battle of Bull Run, was a decisive Southern victory, the Battle of Antietam was the first fought on Northern soil, and could have been a huge Southern victory, but Lee blundered and pulled back and the battle was a draw. The battle led Lincoln to declare the Emancipation Proclamation, which promised an end to slavery at the end of the war. The Battle of Fredericksburg was the final battle of 1862, and the North was soundly defeated, putting an end to the idea that the Union forces were far superior and the war would be relatively short.

Reference

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