[cypher]

First I just wanted to say don't feel bad about not taking the other hamster, most hams fall out & fight within a short while as they begin to mature & females are even more territorial than males so that would have probably ended in you having two hamsters in separate cages anyway, they aren't social animals & don't need the company of another hamster.
If you could post a photo of her cage that might help us to see if any changes there might help.
A lot of hamster toys don't get used, the most important things for them are deep substrate for digging & burrowing, a sand bath with chinchilla sand (not dust) which they just love & is also important for keeping their coat in good condition, a good size house with no base sat on a deep layer of substrate so that they can burrow down underneath, the house needs to be dark so a bendy bridge over the door can help.
A few extra hideouts like coconuts or egg boxes with a hole in each end are enjoyed, bendy bridges can also make little hideouts as well as places to climb on.
Most chew toys don't get used much if at all, some will chew on apple or hazel sticks but almost all go crazy for whimzze dog chews so you could try those.
If you can get her cage set up in a way that makes her feel safe & has things she will enjoy her confidence will grow.
Do you have a playpen for when you get her out of the cage? An enclosed area can work better than free roaming, the hamster ball shouldn't be used with dwarf hamsters, it's too dangerous for them & most are scared of them so putting her in the ball will actually be more likely to knock her confidence back.
Young females are particularly active so don't try holding her too much, if you can sit in the playpen with her, let her climb on you if she wants to, getting her to climb onto your hands is fine & I would encourage her to do that but they don't want to sit still for long at all especially when young!
Give her plenty of things to explore in the playpen, cardboard tubes & boxes can make great toys for them, you could put a wheel in too, let her run around a bit before trying to interact with her too much & just take your time, go at her pace & you will get there.