Janie gave you some great advice. I had a really low weigh gaining DD too. She only averaged 100gm a fortnight and I was so stressed too. Turns out my girl is just a tiny tot and that is all she was going to do regardless. It sounds like you are doing an amazing job feeding/expressing. The amounts you are expressing are fantastic. I was lucky to be getting 40 -50 mls in total. It is so hard but try to enjoy feeding your little girl too. Why don't you have a nice bath together so you can relax and she can have a nice float and a beautiful feed. Or even spend the day in bed together with heaps and heaps of skin to skin contact. It would be beautiful for you both.
Keep us up to date darl and use the support and resources on here.