We all struggle with these issues. It isn't easy and lots of what is happening doesn't make much sense at times. Have you read about detaching? It is difficult to do as well, but it may help you cope.
You can't take care of her. She doesn't really want you to and every time you think of her needs and take care of her needs it is at the expense of your own well being. Start putting your well being first. You need to take care of you.
She is a grown woman and can find her own pair of pants. Be polite and tell her that she may have to look a little harder to find them. If she talks to you disrespectfully, politely respond that you don't like being talked to that way and you will not engage in conversation until she speaks to you respectfully. If she continues, say nothing and calmly walk away.
