Some important thing to consider when pulling heavy loads up hills are to gauge your strength/stamina, the conditions of the road, and the traffic. Where I live, there are many hills, and many people driving cars/pickups who don't give a hoot about a person on a bike with a trailer working their as* off to get to point "B". Don't feel bad about taking a breather after extraordinary exertion, even if it means pulling over midway on your journey.