In the first place, congratulations!
Now I'll try to translate your words in the way I'd say them in Spanish. I'm pretty sure that other girls in the forum will contribute with their own suggestions.
So, you asked if I will be your wife, join our lives together forever? All I can say is...I want this more then anything in the world! you are my world! I love you more then any number of words could ever express! You have brought me more happiness then I ever imagined possible! Thank you for being the man you are! Your beautiful daughter and I are here, waiting patiently for you to come home to us. We love you so much!
yours forever and always,
p.s. remember, when you get home you still have to ask me the "right" way! I don't want you thinking you're getting off the hook that easy 
¿Así que me preguntas si quiero ser tu esposa, si quiero que unamos nuestras vidas para siempre? Todo lo que puedo decir es... ¡Es lo que más deseo en el mundo! ¡Tú eres mi mundo! ¡Te amo más de lo que las palabras pueden expresar! ¡Me has hecho más feliz que lo que jamás pensé que pudiera ser! ¡Gracias por ser el hombre que eres! Tu hermosa hija y yo estamos aquí, esperando pacientemente que vuelvas a casa con nosotras. ¡Te amamos tanto!
Tuya, por siempre y para siempre,
P.D.: ¡Recuerda que cuando vuelvas a casa todavía tienes que pedírmelo "como Dios manda"! No creas que vas a zafar tan fácil
It's not a literal translation, but I think it keeps the spirit of your letter. By the way, please note that you spelled "then" instead of "than".
I used "como Dios manda" for "the right way", it's a very usual expression, and very suitable unless you are a fanatic atheist. If you are, you can change it to "como es debido". "Zafar" for "getting out of the hook" might be an argentinism, I'm not sure, but I can't think of any other equivalent expression. Where is your boyfriend from?
What do you fellow translators say? This is a good example related to what we were discussing in the topic "te quiero/te amo". I didn't hesitate to use "te amo" and "deseo", because it's a very romantic letter. I'd like to hear your opinion.