Why is she like this?

I would suggest a sit down with mom, perhaps mediated by a therapist. It sounds like she has a hard time being emotionally supportive and trying to explain the importance of wanting you to succeed. My mom is the same way and always has been. She yells at me and is very critical but after a long talk with her and my psychologist I found out that she is just really trying to help me get the most out of life but she just has her own way of telling me. She doesn't think she yells or is critical because in her mind she is trying to help. You are still young even though you are a legal adult and she's been through life and most likely just wants to help you avoid the mistakes she made. But the truth is you have to figure out what will work for you and unfortunately that means making those same mistakes. I doubt that she means to hurt you, and you have to vocalize your feelings in a mature and open way. It's easy to want to yell and mom and be angry, but that is only going to cause hurt and confusion and then no body is going to be happy.