If everything had gone right for the North and the South had put up a good fight while going down, I think the Union could have put an end to the Civil War in about 2 years. These things take time.
I do not doubt that both sides produced men of bravery and skill as both officers and enlisted men. That much is obvious. But building an army is not about just that. It is about logistics and organization and experience and training and a host of other things. Simply learning how to keep a large force together in camp is a sore trial: note the massive losses to disease in the opening months of the war, largely preventable with experienced officers and non-coms, training, experience, organization and medical expertise.
In the European militaries of the mid-Nineteenth Century, it was generally assumed it took about six months to train a raw recruit into an acceptable infantryman; 12-18 months for a cavalryman -- 24 months if the cavalryman was a lancer; about 12 months for an artilleryman. That is for armies with a large and experienced organization, used to turning raw recruits into soldiers. The US Army of 1861 is not that. They are only about 16,000 strong on January 1; some 1100 are interned in Texas at the start of the war; about 30% of the officers resign or leave to "go South" when the war comes. Almost all of them are between the Mississippi and the Pacific, and so out-of-reach for months. Experienced officers and men are scattered about to try to get the raw men into some kind of shape, to build organizations, to put the Army together.
Without all the preparation and training, without the support organization, without the experience, a raw army can fall apart and lose a large part of its' strength simply making an unopposed march. Men become sick or crippled up; horses break down and become worthless; troops starve within a few miles of food; ammo is never near when you need it.
That, in a nutshell, is why we see so little in the way of combat in 1861. The armies are training up and organizing. Any attempt at fighting is disjointed and falls into disarray (see 1st Bull Run).
They are still disjointed and mistake prone in early 1862, but much better than in 1861. The early battles show brave men learning the trade in battle -- always a bloody and painful experience (Mill Springs, Henry & Donelson, Shiloh, Seven Pines, the Seven Days, Pea Ridge ...). By the second half, Lee and McClellan have the troops up to better standards and the mistakes start to become battlefield leadership instead of organizational incompetency (Pope and McDowell at 2nd Bull Run, McClellan at Antietam, Burnisde at Fredericksburg, etc.)
In 1863, actual organizationally-competent armies have appeared, often with not-only-brave-but-competent leadership. Now we see battles of operational and tactical skill fought by worthy opponents: Chancellorsville, Gettysburg, Stones River, Tullahoma, the Vicksburg Campaign, Chickamauga, Chattanooga, Mine Run ... Somewhere in here the last real chance for the Confederacy to win disappeared, probably between the moment when Grant crossed the Mississippi and when Bragg failed to do something with his victory at Chickamauga.
After that, the South can only hope for a war-weary North to throw in the towel and stop trying to win. The 1864 campaigns are all about that: can the South hold on long enough to defeat Lincoln at the polls and get a Democrat in committed to ending the fighting (I don't think McClellan would have, but they could hope.)
But in late 1862 or early 1863, it is possible to picture some grand Confederate victory that would have broken the Union spirit: a better Chancellorsville, a smarter Pemberton, a Bragg-led force that actually does something victorious in Kentucky or smashes Rosecran at Murfreesborough, a luckier Lee who does something great in Maryland in September 1862. It just didn't come off. By late 1863 the sheer weight of the Union resources (men and material), organized and applied with skill, was beginning to become too much.
Tim