My 36yo boyfriend was just diagnosed with congestive heart failure and stage 3 kidney disease. How do I pretend I'm okay?

Last month my boyfriend got the flu and we wound up in the hospital because he couldn't breathe. What we thought would be a quick trip to the ER became a nightmare I can't stop reliving. We ended up staying there for about a week, where we learned he is chronically ill. We have been in and out of the ER since then, and he's certain he'll die this year. He even offered me an out. Just after the diagnoses, his beloved cat had a sudden turn of health and passed away in his arms. Understandably, he has become a completely different person. Selfishly, the realization that our relationship will never be a priority again has taken me down hard. Our plans of marriage, a family, a future are out the window. I have been entirely supportive, and there is zero chance I'll take the "out" he offered me. But it feels like I'm grieving someone who is still here. The worst part is that I can tell how tired he is. If not for me, I'm not sure he'd still be here. Is anyone else in a similar situation? How are you coping?