This may be the song you were asking about, tho it was not a Soldier's Last letter... Simply a love affair gone sour!

Ernest Tubb, and others, recorded a song about a Soldier's Last Letter


THE LAST LETTER
Words and music by Rex Griffin

Why do you treat me as if I were only a friend?
What have I done that has made you so diff'rent and cold?
Sometimes I wonder if you'll be contented again.
Will you be happy when you are withered and old?

I cannot offer you diamonds and mansions so fine.
I cannot offer you clothes that your young body craves,
But if you'll say that you long to forever be mine,
Think of the heartaches, the tears, and the sorrows you'll save.

When you are lonely and tired of another man's gold,
When you are weary, remember this letter, my own.
Don't try to answer though I suffer anguish untold.
If you don't love me, I wish you would leave me alone.

While I am writing this letter I think of the past
And of the promises that you are breaking so free,
But to this world I will soon say my farewell, at last.
I will be gone when you read this last letter from me.

Repeat 1st verse.