Hi S.,
I'm so sorry your little one is so sick! OF COURSE you are sad, who wouldn't be? You probably aren't sleeping well at the hospital. Plus you could be into baby blues or postpartum depression, but I wouldn't jump to any conclusions about those until you are back home and everything is more or less on an even keel again (as if that ever truly happens with two little ones--lol!)
They probably have a social worker or counselor at the hospital if you wanted to talk about those issues.
Beyond that..
Can someone bring your toddler to the hospital so you could go down to the lobby and see him/her? whoever brought the toddler could sit with the baby while you visit with the toddler. You could go to the cafeteria for a snack or drink, or find a comfy spot in a waiting room to cuddle, read a book, play a game, or just talk. Or go for a walk if the hospital is in an OK neighborhood. Some hospitals even have enclosed garden spaces, that might be an option.
I hope you are eating well, especially if you are breastfeeding your baby. When my now 14 yo was 5 days old, he was life-flighted from our (then) home town of Magnolia AR to AR Children's Hospital in Little Rock. When the staff found out I was breastfeeding him, they put me on the meal plan there so that my and his nutrition would be good. That impressed me. Even if you have to pay for your food yourself, or have someone bring it to you, eat properly! You'll do yourself and everyone else much more good that way! And yes, I really missed my 3.5 yo and was so glad when he was able to visit. We were only there a few days, so it must be really hard for you, being there so long.
You'll be in my prayers!
K. Z.