I'm so sorry. I could have written this exact post. When my boyfriend's Crohn's was at its absolute worst, he once said that sex would cause him so much pain it wasn't even worth it and that hurt me deeply. I felt so ugly and unloved and guilty, like it was my fault somehow. Even though he told me every day how much he loved me, I still just felt like I was his caretaker instead of his girlfriend sometimes. And that made me feel even worse because I knew he didn't feel good but it's very difficult to not feel rejected and sometimes even resentful when the roles change so quickly in a relationship.
I gently urge you to try to talk to him, calmly and kindly, about these issues. Does he always pick a fight when you bring them up? For me, it took me telling my boyfriend that it was really hurting me when he refused to tell me things or seek help for problems for him to take it seriously. Do you go with him to doctor's appointments? You could also ask him how work is and let him vent about stressful things.
I also want you to know it's okay to be sad. Nobody likes seeing their partner in pain or struggling. It hurts when you're used to one thing and then it suddenly changes. You're not a bad person if you're struggling! I hope things get better for you.