Well, the way it is now, it's not romantic at all! It's rather impersonal, because you say "Sie" to the woman you love. "Sie" is only used for someone you hardly know or who you are very distanced (or just respectful) to (e.g.: teachers, people you meet on the street, officers...). But as soon as you make a proposal of marriage to someone, it cannot be the polite "Sie" anymore.
Perhaps "mi vida" could be translated as "Mein Ein und Alles" (=my one and only / everything). "Mein Leben" sounds a bit stupid, because you cant propose to your life...
Btw, the (usual) sentence is "Willst du mich heiraten" and not "möchtest". It sounds more definite.