Modern fly rod design has developed to the point where a 5 wt. fly rod does cast a 5 wt line better than an lighter or heavier line. You are making casting harder to learn by attempting to cast multiple weight lines on the same rod, that's not what the rod - nor the line were designed for.
You can really cut down on your learning curve by reading the Fly Fishing 101 section right here on FAOL:
http://www.flyanglersonline.com/begin/101/

The heavier and/or longer a rod is the harder it is to "stop" it - it is stopping the rod which makes the line go. A 5 wt rod will be 'easier' to cast than a 6. Glad you found FAOL, there is a lot of help for the new fly angler here *S*