Hi!
So, about three weeks ago I got a new sugar glider named Luna. She’s a youngin’, and last week things were looking.. slow, but kind of hopeful. I have been taking her out in the bonding pouch and bringing her out in the bathroom. In the bathroom she will take treats from me. She’s lunged at me a few times, but hasn’t but me. This past week though, she has been crabbing a lot more and and lunged at me. She’s been skittish, which is to be expected. She still hasn’t let me feed her a treat through the cage bars. She crabs quite a lot, every time I go near the cage she flees to the back corner. ( if she’s awake) if I stay there long enough, she will relax and she has came up and sniffed the treats I’ve offered her through the bars.

I’m worried I’m going backwards, that I’m not doing something right. I know I’m probably being paranoid or something.. but.. I don’t know. I just really don’t want to mess up with her.
Also: this is just something I’ve noticed, she absolutely HATES the sound of zippers and bags ( like plastic bags and stuff)

I'm not an expert but every time I've hit a wall or gone backwards with my boys I start over at the basics. I've been bonding with mine for 9mths and for the 1st couple of months if they seemed to go backwards then I would go back a step to where they were comfortable. Try just sitting next to the cage every night and talk to her and give her treats for the next week or so. Make sure your scent is on plenty of fleece in her pouch too. Then I would move onto giving her treats in her cage and so on. Do tent time every night for a couple of weeks after. Don't bother her just ignore her and read or watch tv. She will eventually see you as her tree. One of my boys would bite me constantly if I tried to touch him so I ignored him and he came to me on his own. Sometimes I could only do tent time for 30 min, sometimes 3 hrs. Mine are both terrified of plastic bags so that's normal. Also live mealworms were and continue to be my best friend for bonding. Some gliders hate even the smallest bit of light so buy a red light bulb, they can't see it. Mine will jump onto me as soon as I open the door now and catch a ride into the tent. I even have one that will put himself back in his cage when I unzip the tent. His brother has to be grabbed with fleece and put in quick or else he will hide down my shirt because he doesn't want to stop playtime.

I think the plastic bag noise could be a natural instinct, like a snake or
maybe something moving in the leaves, so a natural flight kicks in.

sounds like you are doing things right.
3 weeks is still a very short time with some gliders, only thing I would
maybe change up, get dried or live mealworms, you say "she will relax and
she has come up a sniffed the treats." I might, once she is relaxed, slowly
open cage door, move my hand halfway to her slowly, then see if she will come
get the treat. just sit and wait for a few minutes see if she will come to you.
if she just runs and hides, go back to trying to get her to take a treat thru
the cage bars. I also put treats in a container and give it a small shake, this
sound brings 6 gliders to me.