You have already proven you are a lunk head, taking her on a 14 mile hike. Its a miracle that she went a second time, especially with the heavy gear. A third party is the best teacher for S.O.s, and I'd get her a book like Mike Clelland's lightweight backpacking book. That will have great tips for her.
Finding a partner who likes to backpack is the hard part. You could bend over backward to let her have a good time, like having good food, carrying her sleeping bag, letting her use your comfy sleeping pad, doing all the cooking and cleanup. If she likes the activity, it would be worth the money to buy her a decent sleeping bag and see how it works out. For water, take a filter plus a water treatment, and double treat the water. For distances, trade off trip styles, one trip your way (but keep the distances down) and one trip her way. Compromise, be considerate, be patient, accept other peoples preferences and opinions as valid, and don't be a lunk head.