The length of the board all depends on the style of riding and what you need it for. It should be a good length for most riding, especailly freestyle, but is probably a bit small for really fast riding or powder...but not a real problem. It definately wont make it harder to learn...probably easier.
I'm willing to bet that you are just struggling with the stiffness and responsivness of the board compared to the cheap boards you probably learned on while renting. This may slow your learning curve a bit, but shouldn't hurt at all in the long run. Eventually you will be a better rider becasue of it.
Stiffer, more responsive boards just make you use better technique. The soft boards allow for more mistakes and let you use sloppy technique, but won't allow you to progress as much.
Give it a day or two and you will be right back where you were before. I got a new board this year and it took me about half a day to adjust, and I've been riding for several years. You'll love the custom real soon...enjoy!