This is an awesome idea.
Hmm... Well, you want it to comfort, right? Maybe start off with a bit of gentle encouragement, a little nudge in the right direction. Let future-Sammi know it'll all be okay; she'll get through it, and when she does she'll come out stronger.
If you've been in a situation or two where it seemed impossible, but you made it through okay, that might be worth mentioning. Remind future-Sammi that she's been in similar positions before? Then lighten it up with a bit of humour, bring up some happy memories.
I'd start it off with something like,
Hey [compliment].
I heard you might be in a bit of a rough patch. Just wanted to let you know that you'll make it through okay.
And go on from there?
Alaric