Not being that well-versed in the MLP-verse I didn't read the fic, but here's my two cents anyway:

If "I'm afraid of dying" didn't manage the intended emotional appeal, it may be because of those allegations of selfishness you already noted. One solution is to steer attention away from what death implies for her, and towards what it means for someone else. Altruism, if not overdone, should work better than self-interest (however enlightened). Here's an excerpt from one Damien's fanfic Ascension, which I felt worked quite well:

This Saria was just too young to understand. Paige didn't believe she had to explain herself to a child and her biases toward the Kokiri began to surface. "Well, Link is Hylian and he needs a Hylian to raise him and meet his needs. You're just a child, yourself, cursed to be young forever! What could you possibly know about children?"

Almost as soon as the words left her mouth, with a great suddenness the sky opened up and the rain began to pour down on the strange couple. Though her face remained angered, the fear that she was in a very magical place and that she may have over stepped her bounds, was creeping into Paige's bones. Looking at the face of Saria and the tears she was sure that were racing down the child's face lost in the rainwater, Paige knew the skies were mimicking the mood of the Kokiri.

"Is that so wrong?" Saria asked in a quiet voice that despite the roar of the rain seemed to echo through out the woods. "Blacky" the white wolfos, sensing the mood of her friend, nuzzled closer to Saria. "Is it wrong to be a child forever? What is so great about being an adult?" a bite of anger was starting to enter into Saria's normally angelic voice and a peal of lightening boomed from the sky. "Working all day… Worrying about this or that… growing gray, weak, old… Watching yourself and everything and everyone you know slowly decaying. What is so great about dying? I don't want those things to happen to him."