I have a nearly 17 year old who is trying to get her headaches dxd and under control. She's a bright young lady and I allow her to take her own meds. Has she made mistakes or forgotten? Yes, a few times. But so have I with my own meds.
At 21, you are legally an adult and responsible for your own healthcare. She should respect your wishes, and unless there is a reason that you need help, she should let you handle things. There is such a strong love that moms have, and a strong need to take care of and protect their children. It does hurt her that you are hurting and she can't 'fix' your headaches. Maybe the two of you could sit down and have a good talk and find out why she feels she needs to control your meds. Some people are just controlling. Her behavior may make you feel like she's treating you like a child, but I bet that is not her intention at all. Is she really afraid that you will take them wrong? Perhaps she just needs some more reassurance that you are being careful. It's wonderful that she is able to help you since you are unable to be financially on your own, and maybe it would be easier to humor her a little. Maybe there could be some kind of compromise where you are in control of it, and she has a way to check that you are taking it correctly. Maybe if she could explain her reasons behind this, you wouldn't feel so annoyed by it. Sounds like you are trying to show her that you understand how to take them and you didn't make a mistake so I'm not sure why she still feels she needs to control them.
I hope the two of you can talk it out and both of you will be happy with whatever you decide. Good luck!