Took my best friend on a hike

This is Aerith, she's a 140lb Neapolitan Mastiff. She alerts me before I have a seizure, and reminds me to take my medicine if I forget.
She's a good girl.

She loves to pick out JUUUUST the right rock, present it to me, and lick the salt water off it.

She's getting old, I'm worried about what im gonna do when she's gone.

She's been with me through a lot. Moved all over the state with me. She's so sweet I always wondered if she'd even get aggressive in order to protect me.

One day that question was answered. Aerith and I were sitting in a parking lot, waiting for a friend to get off work. When out of nowhere some really creepy guy came walking up fast and my gut twisted and I had a bad feeling. Aerith did too because immediately she jumped up and started walking towards him menacingly and with a terrifying growl. The weird dude had started saying something but immediately he put his hands up and backed away. He asked if my dog was dangerous, and I told him "only when she needs to be." He walked away at that point.

Aerith stayed on guard after that. It was surprising coming from such a sweet creature; I mean, she's an ADA, trained (extremely well) service dog, that loves everyone. Surprising, yet reassuring too.

She gets tired really easy now, I have to start thinking about a new service dog. But my heart breaks at the thought of Aerith not being here someday. Even though I know it's coming, I mean, how are you suppose to prepare for this? How am I supposed to plan on her "replacement" when there's no replacement for Aerith? :'(

Thanks for reading, just needed to vent. Down-vote if you must