I was adopted and when my mom is mad she says that she wishes she never would have adopted me? Does she really mean it?

Sometimes when people are in pain or feel backed in a corner, they lash out. They often grab for the quickest think to inflict damage (mentally). When angry, it pushes reasoning ability to the edges, so though processes get processed by the more primitive survival part of the brain. That is ruled by fight flight or freeze impulses. In this case hers goes to fight. Now, she should have better emotional tools to keep her from getting to that point. But even the best of us can find us in that place from time to time. It is very likely that at some point in your life you will be in a similar place, and you may be shocked you just said something to someone.
What to do about it is a bit of a gray area. If you see it for what it is, her being in a bad place in her mind, and not having the tools to process her emotions very well, then it can be easy to see that what she said shouldn't carry weight. It was chosen because she wanted to inflict damage. If you weren't adopted, it would have been something else, really whatever is handy to her brain that would produce that result. Because it is a dumb tool, what is actually said doesn't matter, therefore it isn't true. It's a tool, a means to an end. When my ex wife used to this, I would look past the words, and see a scared little kitten backed into a corner swiping with tiny paws at a perceived threat. If you can see it this way, instead of taking her words at face value, that will go a long way to your peace of mind.
The second component of this is her. Even if she really meant it, she shouldn't say that. She is not good at processing her emotions. Can she be better? Maybe. Some can, others can't. I think of it as some people being emotionally handicapped. You wouldn't get mad at someone in a wheel chair not walking, if she is emotionally unable to process things like a normal person, getting upset that she isn't better than she is, is only an exercise in your frustration. The problem is you can't see an emotional handicap the way you can a physical one. So by all means, try to push back some, but if you try and get nowhere over time, you may be cautious not to ascribe failure to a lack of ability on her part, or failure on your part. It may be that your expectation that she relate in an emotionally healthy way might be more than she is actually able to perform.
Pushing back... know that it is okay to set your own boundaries. You can say her behavior is innapropriate, or that she crossed a line, or what she said was messed up. Just because someone is an adult, doesn't make them mature, and just because someone is a kid, doesn't mean they don't deserve respect. That said, time and place can matter. As I said about primitive part of the brain, when you are in fight flight freeze mode, you don't respond rationally very well. If she is in the heat of anger, and you provide a completely rational, emotionally balanced plea, it might be like throwing a ping pong ball at a brick wall. It takes time for a person to calm down, and for them to feel that they are no longer under threat by the perceived thing attacking them, before higher level functioning returns. This time frame varies, but tends to be 15 min in general. If you are observant you'll be able to get cues as to when the shift happens for her. This is why it can be wise to make any pleas long after the initial incident. They've had time to come down, and process what they did and said, and they may be able to in that frame of mind regret what they said.

I think your mom is struggling and doesn't know what to say or do. So sometimes she says things that she probably regrets.

How often does she get angry? She might be angry at the way her life has turned out. Maybe she feels overwhelmed by life? Or maybe she's a certain age and hasn't achieved the things she wanted to do. So her anger might be at her life and herself, and not really about you.

If there are times when she's calm, maybe you can sit her down and say, "When you are angry, you say that you wish you hadn't adopted me. Do you mean that?" Be calm. Listen to her. Repeat back whatever she says to her to see if you understood her. Then go off and think about it.

I imagine she might say that she gets frustrated with you sometimes but doesn't really mean it. Or she might say that she imagined it would be different having a girl, and that what she imagined was wrong, no matter who she had as her child.

What does she say to your brothers when she's angry at them? Don't they ever do anything wrong? Does she say, "I wish you had never been born?" Then she might say that to you if you were her child genetically. In that case, it's not really about your adoption, it's that she is having a hard time being a parent.

You can talk to an adult you know about this, like a teacher or an aunt or the mom of a friend of yuors. You can suggest family counseling. You can keep a diary where you talk about your feelings. (You can keep it as a private blog online that's password protected so she can't read it)

I'm sorry dear. I don't think this has anything to do with you. I think your mother doesn't know how to deal with her frustrations. That would be true of any kid she had, not just you.

They are her feelings and she has to be free to have them. I'm pretty sure it has to do with first moms death. We all grieve differently. She might be focusing on this instead of admitting how much it hurts to know she will never see first mom again. Tell the older kids to lighten up on her a bit. She isn't trying to hurt anyone by saying this. She is just in pain and has no idea how to deal with it. If she isn't in therapy it might help right now. Just listen to her and be there.

No. She says that when she is feeling overwhelmed and doesn't know what to do. The teen years are sometimes difficult for mothers and daughters to work through. Both sides can say things they don't really mean. I think it is a cruel thing to say to you, even so, and I am sorry this is happening for both of you.

If you think having the hypocritical opinion of hating the use of the word "ain't" but thinking it's acceptable to use the word "there's" (instead of "there ARE") to describe PLURAL things/people is fine, you deserve to suffer for your support of hypocrites!

She sure does mean it.