Did you knit a swatch and get your gauge for this pattern first? That may be part of your problem. If you knit too tightly, it won't fit.
Also, what type of cast on did you use? I knit a beautiful pair of mittens for a child, only to discover that she couldn't get them on her hands. Even casting on loosely, a standard long tail cast on doesn't work. I now use a German twisted cast on for hats, mittens, and gloves. This is a stretchy cast on. You can find it here at KH.