Hi there, this is very touching, and it does resonate with me.

I have felt a similar way about someone I was deeply in love with many years ago. I didn't think about her death, but everyday I did think, "If she leaves me someday, I will have nothing left, and I will unite with God.". I'm 31 now, and more independent, but when I'm with someone, I'm entirely codependent, as you seem to be, so I understand your predicament and your concerns very much.

I have been in many brief and unhealthy relationships, and have finally learned how important it is to be independent. Because, each time, I would put all my hopes on this other person, and when they'd leave, I'd be left in despair, not knowing how to put one foot in front of the other.

There are a few suggestions I have. I haven't given this too much thought, so they may sound a bit unpolished and even cold, but:
1 - Easier said than done, but I strongly feel, even though I myself tend to be codependent, that, we should all strive to be independent, even when we are with someone. This means - we have a side of ourselves that knows how to take care of ourselves. Our "identity" should never be relinquished, even in a relationship as strong as a good marriage. This also means - we maintain our friendships, our hobbies, our "me time", even while in a relationship. So that, no matter what happens, we are never left stranded high and dry. I think this is essential.
2 - If nothing else, try to take comfort in knowing that there are some things in life you just don't have a choice about. Death and taxes, to name a couple. If you did have a choice, you should be worried, because you would have to make a decision and act accordingly.

So, if you begin to develop this independence today, when the inevitable happens someday, you will be better prepared to handle it, right ? You know you will have to, so it doesn't hurt to prepare yourself now.

Sorry, I know what I have to say is not of much help, but I really think that every person on this planet should strive to be independent in as many ways as possible - financially, emotionally, physically, etc. This is how I see it. I've always been a loner, so that's my perspective.