Thanks for the responses & suggestions of reading. To answer some of the questions...
I am currently going through the process of removing myself from the FOG, I have detached from him to the point where I am able to do this and I am becoming very angry at the way we have been treated. New things are dawning on me all the time, for example I feel anxious & on egg shells with him & his moods, it dawned on me if I feel that, so too would my DD.
His 'off' discipline - One time he tried to implement the naughty step, but pinning her on the step while she fights tooth & nail, screaming 'mummy', it was horrific. (I am not supposed to interfere).
This week he told me we should have smacked her once very hard at an earlier age to instil the fear of it happening again & thus use that fear to control any undesirable behaviour. He has learned this from his childhood & was passing it on as sage advise. Before we had children he expressed no such opinion. He says it is 'too late' now (phew) but...
His expectations of 3 yo behaviour is not realistic imo.
He watched porn on his phone with her present in the room.
He indulges his addictions to check out of family life, he cannot cope with everyday stresses (job, parenting, watching the news, outings, eating a family meal, holidays) without getting stressed/furious & making us all miserable. The sucks the joy out of life.
I have been on anti-anxiety medication since DD's birth & I am starting to think I can't get off them because of him. He is not emotionally supportive of me in times of need at all. e.g. he refused to stop drinking at my due date (he had previously agreed to this), so he would be able to drive me to the hospital & be sober to be a birth partner & deal with any medical decisions. He said he was not going to as I was trying to control him. He used this time when I was vulnerable to make a point and I feel hugely betrayed. Really the list is endless. I realise this is not the obvious & dreadfully sad - 'being beaten up & called vile names' situation we often hear about on MN which is why it has been so hard for me to understand what is happening over the years. I have often thought it is my fault and tried to put things right as you might with a relatively normal partner.
I am writing the letter because it is part of the process the Lundy book suggests. It is an intervention, no holds barred. If he gets angry, so be it, that will tell me something. I have told him that I am near the end with this relationship. This has shaken him up, he has admitted he is an alcoholic & is going to get professional help. He is says he is deeply ashamed. However I am becoming aware just how many problems there are that I doubt he can change enough, quickly enough for me.
But now I am in a bind, the same one all women leaving destructive men are in. If I leave him, my DD has him out of her life for the majority of the time and a safe place at our home (it will certainly be my intention regarding access that she is resident with me for the majority of the time). However, he will have contact with her, and I will not be present - do people see this when they say 'just leave'? So I am currently thinking that it is still worth me going through this process of calling him out on his behaviour and giving him the opportunity to address it & start to make changes, for my DD's sake.
I am aware how difficult change is, so even if he only moves a small way or I help him to be more aware of his behaviour that may bring some benefit for my DD.