I wonder if it might be the age and not your specific child? My DD7 goes through this, it can be the worse day of her life and the best day of her life in the span of ten minutes. When it's the "worse" day of her life she will often say something like that. I just reassure her that we are very greatful that she was born and that we love her very much. These are often times when she is frustrated as well, often with math facts or some other simple thing that she perceives herself as failing in. She tend to think she should automatically know everything or control everything around her which just isn't the case. I have to remind her of this frequently. You are not alone.
