ooooh... I like what Dolly said about putting the letter in a safe deposit box. I think I might be using that one myself.
As far as telling HER.... like the others, I'd say check your motives. Is it true? Is it kind? Is it necessary? I would especially think about the last two... would it be kind to show her how far YOU have come (not knowing your story, I would suppose that she has not come as far)? Would it be necessary for growth or wellbeing for either of you?
I think by "thanking" her, you might be ... just a little... doing a bit of the "neener neener, I made it in SPITE of you" thing. At least, that is how *I* would feel if an old boyfriend/husband showed up to show off his accomplishments.
When it comes to my opinions, I hope you know the drill.... Take what you like...