lol I'll give you exactly the reason why you fell out of love. You won't like it and will probably thumb me down and all that, but it's the truth. To put it as simple as possible without going through a whole slew of scientific terms. You're body got bored of him lol. Humans are special in both an insulting and good way. Humans in the early years are programmed to be whores... This is ESPECIALLY true for women. It is the reason that when you are on your period you see any decent guy and want to have sex with him. You see maybe your mind loved him, but your body only wanted him for as long as he was new. However hormones and pheromones given off by his body during contact eventually begin to seep in and your body rejects them. Those hormones and pheromones are an attractant at first, but they are foreign and your body recognizes them and removes them from the body. Once that happens any contact with him sexually no longer derives the attractant response as your body simply removes them right away. Now I'm sure you're going to wonder how young relationships have worked in the past. Well there is three things that happen. A pregnancy creates a new hormone that encourages the two to stick together for the welfare of the offspring, the man/woman that the person is with is more attractive physically than others within the area, and finally fear. The fear of consequence from leaving the relationship. Now as for why humans are especially special is as we get older our bodies no longer are looking to "breed" with many other people, but instead are settling down to take care of children created(whether or not any children were created). This happens somewhere within the age of 30 to 40 years old. So yes for most of your young life you will continue to have whorish tendencies really do to no fault of your own. You will fantasize about other guys than the one you're with etc.
Now there really nothing you can do to change this other than for him to become more attractive physically or for you to get pregnant(wouldn't suggest that one). Don't worry when you're around 30-40 years old and you're with a guy you will most likely love him till you die. Or you will reach that age and have no one and feel horribly lonely emotionally like my mother for the rest of your life till you die. I'm really hoping the latter doesn't come true for you.