Hi there! I also took a look at your swing. First, mvmac's response is stellar. Study that video and try to replicate the swing you see in his example video, placing emphasis on keeping your lower body more stable during the swing.
Some golf basics: hitting behind the golf ball (ie a fat shot) is almost always the result of not getting your weight over to the left side at impact. By getting your weight over onto the left side before impact you will be promoting a slew of good things, like coming into the golf ball with the hands ahead of the ball at impact and the shaft leaning forward, all things that will promote hitting the ball first and then the ground, and hitting down on the ball.
In your swing what I saw was a tremendous amount of hip rotation in the back swing combined with a sway to the right side. At the top of the backswing your hips are so open and your body so far off the ball that its going to take an incresibly timed athletic move to get back to your address position and onto your left side prior to impact. Instead, think of the golf.swing as more of a rotary action. Your upper body turns away from the golf ball while your lower body tries to maintain the position you were in at address. As you wind the upper body back, try to feel like you are restricting the hips and teyinf to keep them in the same position as address. In actualityx the wonding back of the upper bidy will pull the hips back slightly and you should feel some tension, like your body has been wound back like a spring. From there the correct move is to do the opposite, rotate your hips to the left, shift your body weight onto your left side, and let the lower body and club fall onto the correct plane. From there you can swing hard and through the golf ball. A good drill is to practice the forgoing with 1/2 swing, pausing for a moment between the back swing and downswing.
Hope this helps!
P.s. fatigue will always affect performance. If your arms feel tired and your focus is gone, its always best to move onto chipping/putting.