Well, I can understand some of what you are going through. I say "some" because my 5 year old daughter is not taking it to the extreme that yours is, however, I have a suggestion.
I noticed similar behavior in my 5 year old daughter, so I began to pay closer attention to when it occurred. She is the middle child; I have a 7 year old boy and a 3 year old boy. Middle children usually feel that they are not getting the attention that they need. For us the situation is capitalized because we have girls and girls are a lot more sensitive than guys are. Girls are a lot more emotional naturally. So what I started doing was deliberately singling her out to do special little jobs for Mommy. I would ask her to help Mommy pick out the right outfit to wear to work or church. I would invite her into the bathroom with me while I put on my makeup. I started saying a little chant "girls with the girls and boys with the boys". I went to the store and we picked out a nail polish that we could both live with and went home and painted her nails and toenails in my bedroom. I have asked my husband to just give me time with her alone and take the boys somewhere or keep the boys while she and I go shopping or out for icecream. Well, if you are wondering did it work, the answer is "YES"! She was suffering from middle child syndrome (phrase made up by me).
My husband and I may have an occasional disagreement in front of our children, who doesn't. My oldest is affected the most by these "sad times". So what my husband and I do when we realize what has happened, we make a point to apologize to each other in front of the kids and kiss and make-up. This is what kids are used to. When they have a problem with another child, we make them say "I'm sorry", so why can't adults practice what they preach.
One more thing, be careful about pinning too many labels on your child, i.e. ADHD, mentally challenged, bi-polar, etc. Society will treat your child however you treat your child. Your child will take on the character that you pin on them. For example, if I constantly tell my child that he is dumb and stupid and will never learn anything, then he probably will believe that and won't even try to learn. He will be placed in special classes and forever have this "label" to overcome. However, I have chosen the opposite approach with my children. I put labels on them, but they are POSITIVE labels. I want them to have positive self-images of themselves. I want them to be highly self-confident in their abilities, so I speak that into their lives. I often call my 5 year old "my bestest girl", "my little princess". I tell her that she is so special to me and I love her so much. I don't let her give up easy when she is trying to complete a task. I don't let her say "I can't do it". I get right down on the floor with her and show her how she can do it. I tell my oldest boy, that she is his sister and his job is to treat her like a princess and she should treat him like a prince. He opens the door for her and helps her out of her carseat. He helps her carry heavy things into the house. He watches over her when they are at the park. The reverse is also true, and they both watch over the littlest prince, my 3 year old.
Children are a product of what they see, hear, and feel. WE are their parents and have way more power than we know to help them develop into healthy and happy human beings. I hope my comments and suggestions help you in your situation.