It's normal to feel "down" sometimes- if women only nursed their babies when they enjoyed doing so, there'd be a lot fewer BF babies in this world!! You're not a "bad mama" for resenting the nursing sessions- or even for resenting your children at times. It just makes you human.
As for the change in nursing behavior- it's not necesarily due to lowered milk supply. It could be that your DD is teething or coming down with a cold. Or maybe your supply has finally "leveled out" and she's not used to it yet. Although, it IS possible that the thyroid issues are affecting your milk supply- are you working to get that under control now? It also sounds like your PPD may not be completely under control, and you might need your medication adjusted.
You know all the reasons to BF your baby. You feel guilty thinking about weaning because you KNOW that weaning is the wrong thing to do.