Without more context, I suggest “Ne meurs pas avant moi.“(=> literally don’t die before me) You can’t have a word for word translation because translating I do in French would be redundant and sound clumsy at best.

Now, there are many other ways to express the idea of death. Like :

• Partir

• Quitter ce monde/cette Terre

• S’en aller

• Rejoindre les cieux/l’au-delà

And much more.

But since I don’t know the general tone of your letter, I can’t say if they would be appropriate to use.

Edit

Taking your last comment into account, you can use the expression survivre à quelqu’un to express the idea of outliving someone.

I want you to outlive me can thus translate to Je veux que tu me survives. But Je veux can sound a bit harsh so I would tone it down a bit and say Je souhaite que tu me survives (Je souhaite => I wish).

The exact translation would be: Ne meurs pas avant que je le fasse.

Something you see quite frequently in dramas when someone is dying and their significant other don’t want them to die is:

Ne me laisse pas seul(e) !

(lit. : "don’t leave me alone"). However in your context (you writing to your girlfriend) it doesn’t work unless she is really dying. For your very context there is no French expression that I know that is as common as "Don’t die before I do". You will have to make something up. Anyway all the ways I see to express this idea sound terribly corny, way more than the English expression:

Ne meurs pas avant moi ! (direct translation)

Attends moi pour mourir ! (sounds a bit better imho)