Simply replace the unfilled areas such as "" and "[date]" with your information.
In the time that we have been together, I have been happier than I ever would have expected. You are a wonderful person. You are [adjective], [another adjective], and [adjective]. I have known very few people, if any, as special as you are. I know, without doubt, that one day you will share a deep love with someone who is worthy of you. However, that person isn't me.
When I see you look at me in that tender way, I know you are falling for me. It would be very easy to bask in the warmth of your love. I could fool myself into believing that you have enough love for the both of us, but it simply isn't true. That deep, true emotion just isn't there. You deserve someone whose pulse quickens when you are near. A [man/woman] who can't sleep at night unless you're there. You deserve someone who loves you, someone better than me.
One day, I will see you with someone else and you will be happier than I could've ever made you. I will be jealous of that [man/woman]. Until then, please know that I only want what is best for you.