feeling so down

so my boyfriend of 2 years was recently tested for prolonged qt syndrome. his mom has it and she literally died and came back. she was very very sick for a while. we found out today that he has it as well and i am so sad. i really don’t want to lose him, i’ll lose a part of me as well. i feel like i have to stay up and watch him sleep because his mom said people who have it randomly die during sleep. i’m so afraid. we’ve planned on getting married and having kids and growing old together. i’m not strong enough to lose him