18 days post partum and struggling

I'm really having a tough time breastfeeding. I'm supplementing with formula, saw a lactation consultant and I'm feeding him whenever he wants but I just don't think he's getting enough. It feels like he's always hungry. I'm starting to hate breastfeeding and feel incredibly disheartened and frustrated. On top of this, my husband goes back to work tomorrow which has me feeling weepy and sad. We've had two wonderful weeks at home together with our first child and I'm nervous to be alone. I didn't think I'd feel like this about breastfeeding or paternity leave but I'm just sad. Any advice from others who have dealt with this?