I understand the pain and hurt that you are feeling being faced with a situation where you are asking them for something and they wont give it.... I have been there myself with my parents. You cant choose your family but they are your family for a reason. I found it is part of growing up to manage to distance yourself from needing your parents approval.... you dont need them to give you what you need. You know your painting was nice otherwise why did somebody buy it? It is hard to understand why someone who is supposed to love is is trying to put us down but parents deal with their own deamons and there might be issues your dad is facing like maybe jealousy, his own feeling of inferiority, fearing you are grown up now, not wanting to let you get independent etc etc. It might not be excusable what your parents do but it must be forgiven for your own well being. They are doing their best to their abilities. I am sure they dont want to hurt you or put you down...I am sure they love you.
The thing is unless you settle for what they have to give you and accept them with all their good and bad sides, you wont be able to accept yourself with all your good and bad sides. We are not perfect...we might be lousy parents. you will find the moment you manage to face them with compassionate feelings you will feel a wonderful feeling of peace and they wont be able to hurt you then because you dont need them to do that. The more you hate them the more you will be like them one day..... dont cut them off. They are your blood.