Dear love of mine,
I love you, despite knowing you'll never return those feelings. I'm so happy that, even though we're miles apart, you're still there for me. Even to hear your voice on the phone is such a relief. I know I'll never make you as happy as she can, and it pains me, but I will always be there for you, too. And soon, I'll move on as well, but a part of me will always love you.
....And if you ever leave her, I'll be there for you! I can be just as good a girl for you as her.