Breastfeeding and pregnant

So I am 10weeks and 4 days pregnant with baby number 3, and I have a 6 month old son who is still nursing. My milk supply has decreased drastically and he refuses to drink formula. To top it all of I started back at work with well with hardly any milk stored. I’m beginning to feel defeated and guilty because I’m clearly not making enough milk. My son eats solids and drinks water as well but he constantly wants to nurse and is beginning to get frustrated because hardly any milk is coming out. Any tips for this stressed out mama?