Relief as a feeling?

Hi, working on relationships this month and I’m stuck. I’m working on someone in my family who is an active addict to hard drugs. This has caused me a lot of suffering thinking they should be different and they should stop, etc. When I was working on the section about what i would think/feel if they followed my manual (which includes stopping doing hard drugs) one of the main things I felt was relieved, b/c my thought was ‘they won’t die of an overdose’. I’m getting stuck on how i can think this thought without them following my manual. (b/c brookes offering is that i can choose to think/feel all those things even if they don’t follow it). Do you have any suggestions? thank you