You are not a bad mommy. The fact you love her enough to be concerned about her, that you come here to learn how to best take care of her, and are willing to get her medical attention makes you a good mom. No one is perfect, especially when starting out with any pet, or child for that matter. You learn along with them, and they give back love and a lifetime of guilt. A parent who doesn't worry or have some things they feel a touch of guilt over is probably a parent that doesn't care.
Treat the mites first. Then consider ways that you could possibly work on giving her more cage space, or more floor time. There was someone on here who discussed having a cage that was actually half under the bed to make more room. Maybe with a little creativity and some advice from the experienced piggy parents on here, you can find the room she needs and maybe even enough for a partner.
I'm a first time piggy parent and am learning so much I never knew or even suspected. The fact your little one loves you as much as she does instead of running and hiding shows that you are doing some things right and you do show her love.