Be good to yourself & call it "failure to thrive" or just "bad luck" instead, unless you can point to some specific way you failed her, & note, I'm not asking. But your other snakes being healthy suggest that it wasn't your doing.
If you go to the nearby show in March, it's very possible he'll offer you a deal on another snake- at least, that's what I'd do in his place & under such circumstances. Only you know if you'll feel comfortable with that, & at that time. But it's something to think about ahead of time. I'm glad you let him know & got his answer to ponder meanwhile.