It seems that love is selfless and lust is selfish. If you have doubts, you are probably correct in thinking that lust may have come into the relationship.
Basic morality teaches that people are ends in themselves, never to be only the means to an end. If your focus is on acheiving the end of gratification, you may have to rethink your actions. You can have goals (marriage & children, a stable happy life, to heed God’s calling, etc.), but you cannot morally treat your loved one as a means to these goals. Treat her as a person and hold in your heart the desire to see her better in life and in eternity and you will have love. You can share goals and participate in a plan for life with her or hold hopes in your heart, but using her to acheive them is not moral. Hopefully, with prayer and honest self reflection, you can discern when you are using her. It is not always cut and dried. A good rule of thumb is not to abridge her free will and ability to make moral decisions with manipulation, deception, or force.
That being said, you may be in a passionate and increasingly physical relationship. She may desire this state of the relationship. You can morally not participate in sin. Placing her “good,” as best you can discern it, above your own or her physical desire is an act of love. The hope for a well formed relationship rests on the foundations you are setting now. Isn’t real love the best foundation for a spiritual union in a sacrament ordaned by God? Lust and even great affection are commonly substituted these days, but the failure to start relationships and marriages in love has obvious results in our society.
I got some good advice a while ago, whenever I am upset with my wife, I do something for her. I usually clean something. It has turned many situations to the better when they could have built up and caused fights or resentment. Even when we fight it lasts much less time and we most often agree it was just a mood for one or both of us and it seems silly a short time later.
