(((((Trembling)))))
I am sorry for your situation trembling. Truly I am. I grew up with an alcoholic father, so I know about not being able to get through the alcohol-mist-brain to the wonderful person inside. I eventually gave up on my dad, and led my own life. That was my self-preservation. Not long afterwards he has the most terrible of motor accidents. He literally died twice, he went into cardiac arrest, and the paramedics revived him both times. He spent months in hospital, and that was his turning point in life. Thats what it took for my dad to eventually stop his drinking.
I am praying that your fiance doesnt have to hit that low for him to see that he has to stop. Lets be positive that for him, the very real chance that losing the best thing that has happened in his life - YOU, will jerk him back into reality. Because, I'll be completely honest with you, living a life with an alcoholic non-controlled PTSD sufferer, will not be a life for you, and I cannot see it lasting. Sorry Trembling.
That is why I am so glad to see you looking out for yourself, and putting him behind you for even just a short while. It is vitally important that he knows that you WILL be there for him when he is sober and getting his PTSD treated. But it is also just as important that he knows that he cant act like a brat and jump off the wagon when he feels like it, and expect you to pick up his pieces. There have to be boundaries, and that has to be one of them. That is vital for your sanity. I would tell my sufferer exactly the same thing were my feet in your shoes.
We love our sufferers with all our hearts, but there has to be a limit, and you set yours where you feel you are comfortable. Then you make sure that he knows EXACTLY where they are, and then stick to them. Never waver, or he will take advantage of that weakness. Stick to your guns, and be true to yourself. If he is as serious about this relationship as you are, he will come around, and you will get your "Happily ever after".