Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of love interest],
I never realized that being apart from you for so long would feel this way. Of course, I always heard the old saying about absence, hearts, and fondness. But they never tell you that if your fondness can't possibly grow anymore, that the absence may split your heart in two.
I find that being away from you is almost unbearable, and I know that I can't be away from you often. In fact, I would prefer never to be away from you again.
Will you marry me, and be with me forever?