Tongalee, I tend to think giving the letter to your T for safe-keeping over the holidays is an excellent idea. It's much better than getting rid of it, like you did with the deleted emails, which you might do impulsively if you found yourself getting too attached to it, reading and re-reading, and generally acting in a way that's not really what you want.

Giving it to your T for safekeeping is an important symbolic gesture. The letter is important, and you want to keep it safe, but not to obsessively cling to it like a magic talisman. Because what's really important is the relationship between the two of you, not just the memento she gave you.

But ... well ... it will be nice to have it back sometime in the future. If you explain it to her I'm betting she'll feel touched by your action, not in the least bit offended, as long as you explain your very sound reasoning. It makes really good sense to me.