Incest Survivors Support Group
Incest refers to any sexual activity between closely related persons that is illegal or socially taboo. Consensual adult incest is very rare. Incest between adults and prepubescent or adolescent children is a form of child sexual abuse that has been shown to be one of the most extreme forms of childhood trauma. If you or a loved one is a survivor of incest, join the group...

I felt the exact same as you; I could feel the negative emotions, but not the happy ones. I did try taking anti-depressant medication, but it seemed just to reduce the emotions I could feel anyway, not help me to feel the happy ones. It didn't really work how I wanted it to, although some people say it works for them.
My advice is just what loum said, be gentle on yourself. Emotions are not under our control like some people think. They seem to rise up from somewhere, and though you can push them down, you can't make them come up. You probably have some memory-you that is clamped down hard on the joy feelings, not letting them out. She's angry and hurt, I'm thinking and she feels like no one is really listening to her. I've had a hard time feeling validated, except in this group. People here can accept all your emotions, and that helps a lot to get them flowing.
Good luck to you, you sound like you are on the right track.
Hugs.