My daughter was a premie and spent a month in NICU. Thank god it wasn't longer. My heart goes out to you. I remember how tiring it was going back and forth, pumping trying to nurse, being stressed over her health and well being. Feeling guilty for not being there with her 24/7.
Does the hospital not have a pump in the NICU? Ours did and I would visit my daugher and then pump immediately after. Go back to her isolette and visit a little more and then leave (for food, a nap, a shower, let her sleep etc.) The hospital grade pumps are stronger and will bring up your milk production. Use it if it is available.
Have you done any Kangaroo care? Just having the baby against your skin will encourage milk production. And it's beneficial for the baby too. They did warn me not to over stimulate her by looking directly into her eyes or caressing her too much. Just being skin to skin seemed to sooth her.
We will keep you in our prayers