It was like I was not in my own skin, like I have a split personality...
In a sense you are. You are dealing with a malady which will 'make us' do incredibly insane things. Not least, continuing to use/drink when every fiber of our being insists that it will do us no good. And often we will get high/drunk just at the wrong moment, usually when we have a lot to loose by doing so. It is the 'insanity' of addiction. And you are not alone, it is the common thread (amongst many) which we all have. I certainly did. In fact I still do. I will be prone to having the occasional compulsion to drink, perhaps for the rest of my life. The ONLY think you have to concern yourself with is staying clean and sober. Make that your life's mission.
With regards to not completing things. I once read that people who start something, but do not complete it do this for a psychological reason. I used to paint a lot. However I would hardly ever finish my paintings. I read that by not finishing them.... and trying to get them seen by a wider audience, I was protecting the fantasy that I could be a successful artist. If I finished my paintings, and they were rejected I would begin to believe that I was not talented enough. However, by never finishing them (or attempting to have them seen by people), I could retain the fantasy that I 'could have' or 'might have' been a great artist.