Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Affectionate Nickname],
This morning, when I woke up alone in my bed, I couldn't shake the feeling that I had misplaced something important. When I came home after [activity during day], the feeling was made worse by an unexplainable, melancholy loneliness. Unexplainable, that is, until now.
Leaving you this evening gave me the clarity I had been lacking all day. When I wake up in the morning, there is something vital missing. You aren't there. When I come home after a long day, I can't be greeted by your [adjective] face. After we've spent a wonderful evening together, I still have to go home to an empty [home type]. I fall asleep to silence, instead of [listening as you tell me/ telling you] about all of [your/my] hopes and dreams.
I simply can't take it anymore; I need you here with me. It isn't a home unless you are there. Please, say that you will move in with me.