I'm sorry to hear your loss. When I first saw your avatar, I thought the photo resembled my dog a lot. She is also 15 years old now and we're beginning to get worried to every little thing.
15 years is a long time, but when you have such a great companion, it just doesn't feel enough. Still, I think, Mysti and you were lucky to have known eachother. I hope another little rascal can steal your heart soon, and you can raise her (or him) by telling great stories about Mysti.