Two suggestions, one of which has been made already:
1. Load the mag and leave it loaded for a few days. This loosens the spring.
2. While Glocks come with a mag loader tool, I don't care much for it. There's this fabulous little tool made by a company called HKS (they're famous for revolver speedloaders) which helps you load hi-cap magazines easily. It costs around 8 bucks, and allows your thumb to push down a lever which squeezes the rounds into the mag. They are gun specific, so make sure you buy the right one (the models are listed on the back of the package). I bought one just to try for my Beretta, and now have several for various guns. Trust me on this.