I hate this manga trope that always one person involved is like "I love you but I can't make you happy so I'm gonna reject you" like I know you think you're doing the best for them but seriosuly. Who the hell is you to think you know what makes me happy, thats up to me to decide.
Oof that's rough. If u think of 6 years to live as an equivalent situation, I think most of us would feel conflicted about getting into a relationship that might end up hurting the other person at your death.