Boss Rant!

So I have been working at an Eldery assisted living as an activity aide. Just recently our coordinator just up and left us. So I forced to step in and take the roll. I love my job I love what I do. I get to connect and build a relationship with my residents. As a coordinator I had to over see the flow of things while my boss was away, I had to make sure we had the right materials and see what we needed. Assign tasks to the other activities aides. So basically I'm her right hand and go to. She has a tendency of pointing out all of our flaws and constantly just put us down. Of course she doesn't do it in a rudely manner so we can't go against her. Today I just had enough of her rudeness and her constant put downs. She told me I wasn't as good as the girls she had before and I needed to step my game up. I felt like I did my job pretty well and I am giving it my all plus more. I can't afford to lose my job but I can't keep working with someone that doesn't see the good when it's good. But constantly is always trying to find something wrong. I went to the head boss and explained everything to him on what was going on. I explained to him I love my job and I love what I do and I have grown to call the people at my job family. I agreed to sit with my direct boss and talk it out but at this point I can't take the stress and anxiety anymore. I asked the big boss if things don't work out for me in activities could I possibly switch over to nursing and he said absolutely. After thinking long and hard all day today I finally just came to a conclusion that I am done. I don't want to be disrespectful or hatred. So I want to figure out how to say to my boss that I no longer want to work for her and that I want to go to nursing. Any advice on how I should go on about it telling her?