Warning: depressing post ahead

I think one of the most heartbreaking things I've had to deal with in a while was Sydney tonight realizing that one day Oliver won't be with us anymore, and me then having to comfort her over what will surely be one of the worst days in my life. Weirdly, and I'm sure it's a phase kids go through, she's asking a lot of questions about dying lately. Like, when she's 100, will Brian and I still be alive? Or when will we die? Or, horrifyingly, will I cry if she died? Or what will we do if Brian dies? I mean, for me, as someone who is terrified of death, having to answer all those questions is akin to torture of the most heinous kind.
But I refuse to lie to her, especially now that she's old enough to remember the things I tell her, so when she asked if Oliver will ever die, and if he'll live to be 100 years old, I was gently honest about it. And having had that conversation, she moved over on the sofa to hug Ollie, and then, next thing I know, huge, huge tears are rolling down her cheeks, and she's sobbing about Oliver eventually dying. Of course, I consoled, and explained that it isn't anything we need to get upset over now; we still have several years with Ollie, and that we should be hugging and loving our boy instead of crying about something that won't happen for a long time. She rolled with that, but still was upset, and gave him sad eyes and longing looks all night.
Having settled her down though, I was struck with the knowledge that at some point, I'll have to deal with the loss of my first baby by supporting my second and third babies through their emotional crises (as well as my husband's) first, and then my own crippling sadness about it second. That seems incredibly unfair to me, as I know that on that day, and probably many, many days after, I'll just want to curl up in the fetal position on my bed and cry.