永遠に、いつでも
Eien ni, itsu demoBanzai.
We know it best as a battle cry, but it translates as "10,000 years" or "may you live for 10,000 years". I don't think I'd use it in a love letter, though.
Hmm. Maybe you could. After all, the three ships of romance are friendship, courtship and battle ship.
私 は して いません。