Deciding when to stop.

I had the hardest time with breastfeeding and I am still having a hard time exclusively pumping ( I only get about. 12-16 oz total a day. Baby drinks about 30oz a day). If it wasn't for my mother visiting, I probably would have stopped pumping already. She helps with the baby while I pump, she leaves in 2 months and I don't know if I should start planning to stop for when she leaves. My baby will be 4 months.
How do you come to the decisions of stop pumping? And when you want to stop how to do go about it?