Hi there, You're not done yet. There are still people to meet, places to see, experiences to have that you don't even know about yet. You are wanted. There are people that want you in the world, and their world is a little better because you are there.
It WILL get better. The right now may be dark and feel endless, but it will be short lived and you need to be here for what's next. Life is small steps. In the words of CS Lewis"Onwards and Upwards, further on and further in"Don't loose hope and focus on the small victories, they can add up to amazing things.
Write your own letter soon, and help someone else know they are Martin loved and the world is a better place because they are still here.