I have BTDT. Fortunately, the hosp my DD was tranferred to is VERY pro-bfg, and they helped me a lot. They have a pumping room and freezer storage available right in the NICU, and they strongly encourage moms to BF.
If you can pump and/or store bm at the hospital while you are visiting, I strongly encourage you to do so. If they have lactation consultants on staff, consult them and take advantage of the fact that they are there to help you and advocate for you.
Do you have a hospital-grade breast pump? That is the only
kind that ever worked for me. You can rent them from medical-supply stores and sometimes from the hospital itself. If you are willing to raise complete hell w/your insurance company, they WILL cover the cost; even though they will insist that they don't cover the expense of renting a breast pump. (I fought that fight, and won.) Get yourself a small, good-quality cooler and ice packs to transport the milk back and forth (and storage bags/bottles, if the hosp doesn't offer them). If you're storing milk at the hosp, be sure to bring any extra home with you when baby leaves!
Above all - and this is the hardest part, esp when you are traveling back and forth - keep yourself as healthy as possible. Get all the rest you can, eat well, keep your immune system in good shape (Emergen-C is a good supplement for this). Your stress level will affect your ability to produce milk. When you're pumping, try to think about nourishing your sweet babe.
I have been there, and I understand your feelings and worries. Feel free to PM me if you want.
Best wishes to you and your baby.