-- Last Updated: Feb-21-13 9:15 AM EST --
The stern is naturally much less "locked in place" while the boat is moving forward than the bow. To make matters worse, once it starts to wander or "skid", that motion tends to amplify. Planting the bow deep in the water as well aggravates the problem, as it makes the boat tend to "trip over itself" and tumble out of line.
The handling probably would have been better with you in the stern, BUT, overly bow-light isn't ideal either. Whichever person is in which end, you probably could have gotten things balanced pretty well by shifting your gear much closer to the end having the lighter person. If your girlfriend is more skilled, I'd put her in the stern and shift the gear packs toward her end.
Think of the canoe as a teeter-totter. A small child and an adult can balance a teeter-totter if the child sits at one end and the adult shifts away from his/her end toward the center. If those same two people had a load of gear piled on the teeter-totter, they could simply shift that material toward the child's end, and the adult would not need to shift toward the center. Do the same with your gear load, thinking of the center of the boat as the pivot point of a teeter-totter. "Slightly" stern-heavy is usually okay, but for basic cruising, try to get the boat level. For sustained headwinds, bow-heavy helps, and for a sustained tailwind, stern-heavy makes things easier.
20 mph is a strong wind to be paddling in, and if your girlfriend is skilled enough to keep a squirrely back end under control during most conditions, it might not be possible in that kind of wind. The wind direction relative to your travel direction makes a big difference too. When the stern is too light, any cross wind, tail wind, or some combination of the two will be difficult to deal with.