You remind me so much of myself when I was a young girl. (I'm 28.) I too believed very strongly that God was calling me to become a religious - I believed He was calling me to enter the Missionaries of Charity (the order Blessed Teresa of Calcutta began).
When I was 16, my grandfather died and my father was diagnosed with cancer. I began to realize that God was calling me to be a nurse, so I entered a nursing degree program. Today, I am finishing a master's degree and teach practical nursing.
Eventually, I began to realize that God was calling me to marriage, that His plans for me were different than what I thought they were. 20 months ago I married my soul mate. He is a devout Catholic convert with a 8-year-old daughter from his first marriage (annulled) who considers me a second mother.
I would encourage you to continue praying and discerning God's will. I believe that the reason God decided to call me to marriage is due to a quote I heard once: "The monastery of the 21st century will be the family." I don't know that God necessarily changes His plans for people, but I do know that I have been called to be a good Catholic mother and raise my children in the faith.
Incidentally, marriage is not a lesser vocation than the religious life. Both are equally important - their just different. I will keep you in my prayers, and I encourage you to continue your discernment process. :gopray2: