I think you’ve answered your own question several times, but I will play it back for you. Let’s begin with you “hate your life.”
Each piece of information you have offered comes back to the fact that this isn’t at all what you have signed up for. What you love in your fiancé is his potential, not the reality of his situation. You love what he could be if….
From your description it seems clear that these children will become your responsibility. The mother has abdicated her duty, and you will become the primary female, if not the primary adult in their lives. Do you want this? It sounds like that not only includes emotional and intellectual support for them, but also financial. If you are willing to do so then make peace with your decision and find ways to love them, cherish them and help them. But if you can’t accept this responsibility, have an honest discussion with your fiancé about your limits. Don’t hold back. Tell him what you are and are not willing to do. This discussion will help you decide if you want to add the legal commitment of marriage in extending your relationship.
The military has some wonderful counselors for the family. I strongly recommend making the connection to one of them if your decision is still unclear.