If you have allowed Isabela to keep the relic, the letter will read:
I have the relic, and I am gone. I'm sorry it has to be this way. You've been a loyal ally, but this is best for us both. You promised me the relic, and I know you'll fight Castillon for me, but I don't want this. I've dragged you too far into this mess already.
You don't have to forgive me, but I hope you understand.
If you have told Isabela to turn the relic over to the Qunari, the letter will read:
I have the relic, and I am gone. I've lost too much over this blighted thing to let it go again. I know it would be noble to return the relic to the Qunari, but that would require a better soul than I possess.
For what it's worth, I'm sorry I lied to you again.