I think I have most of it.

It's very difficult to read, as a mother, let alone to labor over the meaning of the words.

And besides, my French is very rusty.

I've put things I had difficulty with in brackets, and I did take the liberty to make what I think are colloquial exchanges, to make it flow better in English. Please correct my mistakes.

The French:

"A toi, petit Papa, si je t'ai fait, ainsi qu'à petite Maman, bien des peines, je te salue une dernière fois. Sache que j'ai fait de mon mieux pour suivre la voie que tu m'as tracée. Un dernier adieu à tous mes amis et à mon frère que j'aime beaucoup. Qu'il étudie bien pour être plus tard un homme.

17 ans et demi ! Ma vie a été courte ! Je n'ai aucun regret, si ce n'est de vous quitter. Je vais mourir... Maman, ce que je te demande, ce que je veux que tu me permettes, c'est d'être courageuse et de surmonter ta peine. Je ne peux pas en mettre davantage. Je vous quitte tous, toutes, toi Maman, Séserge, Papa, je vous embrasse de tout mon cœur d'enfant.

Courage !

Votre Guy qui vous aime"

My attempt at English:

To you, Papa, [si je t'ai fait: I think this has to do with the very familiar way he’s addressing his father], also to Momma: with great sorrow I greet you one last time. Know that I have done my best to follow the path that you have marked for me. One last farewell to all my friends and to my brother, whom I love very much. [May he study well so he will become a man later.] May he learn the lessons that will, as he gets older, make him into a man.

Seventeen and a half years old! My life was short! I have no regrets, except for leaving all of you. I am going to die… Momma, what I want from you, what I want you to let me have, is for you to be courageous and overcome your sorrow. I can’t say [put down] more. I leave you all, all, you, Momma, Séserge, Papa, hugging you all [embrasser = to kiss, to hold, to hug] with my whole child’s heart.

Be brave!
[from] Your Guy, who loves you all