Firstly I would put aside the fact you are not getting anything with a pump. That can be caused by anxiety and is common even for relaxed mums. It does not necessarily indicate supply problems in general - breasts can just become accustomed to baby's milking action and won't cooperate with a pump.
At 19 weeks it is normal and correct that your breast being to feel more 'empty'.
This is because they are becoming adjusted to feeding and are less engorged with blood and fluid around the milk- producing cells. Milk isn't what makes us feel full. It doesn't all sit in the breast before a feed - it almost all made by the body
during a feed. It is also very very normal for women to stop 'feeling' letdown reflex - some mums never feel it. I stopped feeling it after a few weeks too.
It is right that every feed of formula means less milk is made by your body. I would continue dry pumping even if nothing is coming out or learn about hand expressing:
http://www.lactationinstitute.org/MANUALEX.htmlWhat you do pump, store and only give it if she is still unhappy after a feed. Don't ever give it at the beginning of a feed.
Your description of green poo and a baby bobbing on and off can actually indicate an oversupply as well as supply problems. I would really recommend you speak to an expert if you are committed to breastfeeding and want to avoid moving to formula. A very short phone call could answer all your questions. There are numbers on this board or look for your local La Leche League. You've done really well up until now. You've got past the bit where most people struggle you can get through this too.