3 weeks is not very long. She hasn't gotten acclimated with all the new sounds. smells, humans......everything.
Gliders really aren't aggressive even if you perceive it as aggression, it's fear. You are afraid of her and she is even more afraid of you.
Confidence is key with gliders, if you are scared she will know it and it will increase her own already high fear. If you are confident around her she will be less scared and as your confidence grows her fear will decrease.
Yes you do need to take them out regularly but you haven't had time to make a solid routine of it. Once you make it a routine they will quickly figure it out.
Try doing things every day at the same time. Treats at the same time, get them out at the same time, feed them at the same time, take out their food dishes at the same time... When YOU make it a routine, they start to understand whats about to happen whether its you opening and closing the door without interacting with them, or you opening the door to take them out etc... They will learn your rhythm, the sounds just before that thing happens, your voice, etc... once they know whats about to happen they don't fear it and start crabbing less.
Gliders aren't much different than toddlers....that just never really grow up. They thrive on routine and can surprise you with their progress when you stop expecting them to progress. The glider that is calmer with you is more confident than the crabby scared one. Use the calmer one to teach the scared one that your safe. When they are back together the scared one will watch the confident one and take her lead eventually
Not sure who your breeder is but I disagree. you can always take a step back and start over with gliders.
IF the gliders aren't responding positively take a step back, think about how YOU are doing things that YOU can change to see if that gets a positive response. Every glider is different and will need YOU to do things differently for THAT glider. What your currently doing is working with one of them but not the other, so change how you address her and do things. Take a deep breath, clear your mind, don't expect a bad outcome before you start and decide that YOU CAN DO THIS. Show that glider that she can have confidence IN YOU.