Ah yes, emotional blackmail: it can be such a potent weapon.
You've said before that you live in Iowa, which means you became an adult at age 18... so you're entitled to do what you want with your body and whilst your mother can (and will!) always have an opinion on such matters, her opinion doesn't actually carry any weight. Nor should it stop you from doing what you want to do with your life.
So why is she so upset about you piercing your ears? Well, she has probably spent the last 6 years clinging onto the hope that you're just going through a phase and that if she gives it enough time (and denies you for long enough), you'll get over it. But piercing your ears is a permanent thing, so she probably sees it as a definite step towards transition and that's what's hitting her hard.
When you started therapy a couple of years ago, I daresay she hoped that the therapist would 'fix' whatever is 'wrong' with you. It probably never occurred to her (or she didn't want to contemplate the possibility) that therapy could help you come to terms with being trans and would give you the courage to move towards transition, rather than steer you away from it like she no doubt wanted.
Bottom line: she's scared.
She's going through the stages of grief: she's grieving for the idea of who she wanted you to be, and she's trying to cling onto it. You, on the other hand, know that what she wanted for you is incorrect and you urgently need to head in the right direction. By all means acknowledge her emotional reaction, but make it clear that you need to take action now. As an adult, you're entitled to see any doctor you want, and have any treatment you want, without parental consent. You asked your mother about insurance in September: she's had 5 months to think about anything she needs to think about, so now she's just stalling. So call her bluff: ask her if she has any specific questions before you book your doctor's appointment; suggest she attends a therapist appointment with you; print out a bunch of resources and hand them to her. If she can't come up with any legitimate questions or says she needs more time, just say 'OK, if you don't have any questions I'm booking a doctor's appointment today, and you can come back to me when you think of something'. If possible, get your dad involved, particularly when it comes to health insurance.
Then as long as you're able to use your family's health insurance, just book your own appointments to see the doctors you need. If you can't use the family's health insurance, see if there are other ways of financing the treatment you need (can you go to Planned Parenthood, for example?). It's none of her business (or anyone else's) what you're seeing the doctor for or what they prescribe for you. You're an adult now, so that information is private.
If appropriate, come out to your siblings and extended family so your mother no longer feels the burden (and power!) of hiding a 'dirty secret'; take that power into your own hands. And do consider telling her what you told us: that it's been six years since you came out and the problem is only getting worse - and the longer you leave it, the worse it gets. Tell her that these unnecessary waits are putting your life and health in danger. Tell her that her behaviour is making you feel uncomfortable and unsupported. Tell her you're going through a huge, traumatic, life-changing event and you'd much rather do it with her by your side. And tell her that you need her love, support and understanding - and that her resistance is only hurting both of you. Consider telling her that within the next 5 years you'll be fully transitioned and happily living in your new role, and that as you both look back at this point in your lives, she'll wonder why she made such a big fuss when the end result has made you a happier, more fulfilled, more wonderful version of yourself.
Whether or not you start HRT is not your mother's decision. But as long as she perceives you as a child, she'll continue to think it is.
