Trying I'm going to speak plainly & I hope it doesn't upset you, I've been where you are & I know how hard it is to tell him to leave & the fear of the immediate repercussions 
this is not a time for soft words or for my pussyfooting around you, but I'm certainly not trying to upset you
OK, if you are going to die in the next few years then your DD needs a father who is not addicted to drugs, as when someone is in the grip of an addiction your DD will always come second to that addiction, as you are both coming second now. If you are not around to fight for her or to act as a barrier between his behaviour & her then she essentially has no protection from the behaviour of an addict; those godawful morning bad shitty moods, the reckless dangerous driving, the constant excuses & letting down etc etc (Unless of course he will not be the main carer when you have gone)
Remember the three C's, you didn't cause it, you cannot control it, you cannot cure it. You took him back in good faith that he would get some help, it doesn't make you weak or foolish that you believed him but its fairly clear that he will allow the status quo to continue if you don't draw the line somewhere. You need to draw that line right now.
Having a speech in your head is a good idea as you will have some idea of what you need to get over to him, but bear in mind that he won't be listening to it, not at first. Keep the speech on the back burner & just practise saying a sentence, my sentence was to my ex that 'I'm not in love with you any more, I don't want to be married to you anymore, I want a divorce.' Try something similar that's short & that cannot be misunderstood.
If his immediate reaction is violence then call the police.
If he starts with the begging/promises/guilt then just stay strong & if you have to, just keep repeating no. In my experience that first conversation is not one for explanations or understanding anything, it will be about first reactions, so a shocked disbelief, possibly a fight or flight thing, possibly his first response will be to indulge in his addiction. He will blame you, he will say dreadful things but it doesn't matter, he is no longer your problem. He can say what he likes, your decision is made.
If you can, then get angry!This is not what you & your DD need, he had the opportunity to step up & he didn't, so that's his chance gone. You are not a doormat! You deserve more respect than this, illness or not & he is not giving you what you need so you are more than within your rights to get rid. Throw him out! Shout at him! Get angry as the anger will keep you strong.
Keep posting as someone will always be here to read it.
If it helps, then my leaving my exH was a turning point for him. Not immediately no, but a year or so in he decided to get clean, he did a detox & he's now clean & sober & a 100% better father. But it took losing his wife & 2 sons to do it. You hear talk about a rock bottom & perhaps it will be the same for your husband, I don't know.
But you know that living with an addict is exhausting & relentless, once you are clear of that & once he has gone then you will get a relief & a weight will just lift from you. Your life will get much much easier on a day to day basis & you will realise that you are not responsible for his happiness & that he has his life to lead & its his responsibility where his life takes him. You take control of your life & yes its hard, but once you are out of the woods then you will look back & wonder why you stayed for so long. You can do it, I know you can! I have faith in you & you are stronger than you think.