How do you express I love you in Chinese?

I recently traveled to China with my girlfriend, and we had an amazing time exploring the beautiful landscapes and vibrant cities together. As we were walking along the banks of the Grand Canal, she turned to me and said something in Mandarin that I didn't understand. She wanted to tell me how much she loves me, but she spoke only Chinese. I felt a little lost, but then she smiled and repeated it in simpler words. We both laughed, and from that moment on, we were more determined than ever to learn each other's languages. I've been trying to learn Chinese ever since, and one of the phrases that has always puzzled me is how to express 'I love you' in Mandarin. Can anyone explain to me how to say it in Chinese?