If I was going to write a letter to the younger me, it would look a lot like this:
It's 2010 now, and I'm living the life that, right now, you don't believe you'll ever have. I have a job I really enjoy, a solid group of fantastic friends, and the most wonderful man to share my life with. I'm the happiest I've ever been.
You have a long journey ahead of you but soon enough, you'll get here too.
I only have one piece of advice: COME OUT NOW. I have very few regrets in my life, but at the top of the list is waiting so long to come out of the closet to those who are important in my life. And here's the scoop: your family will still love and support you, your friends won't ditch you (and for the few that might, you don't really want them around anyway), and you'll feel a million times better without having to lie to those you love. The girl you're dating will be disappointed that her vision of the future isn't going to pan out, but she deserves to know the truth and you'll both be much happier afterward. And your cat won't care either way. I know it doesn't feel like it now, but being honest with the people in your life will be much easier than you think, and it's the most liberating feeling in the world!
I wish I could impress upon you how important this is, but I suppose everyone comes out when they're good and ready... and it's going to be a while before you feel like you are.
I'll see ya when you get here. :o)