I would stay start small and positive. It sounds like you are really down on yourself and that can get unpleasant if you end up journalling a lot about the things you think you did wrong. Maybe limit yourself to three things you want to change most, then look at what the obstacles are to those changes? For me it's exercise- a really big thing I need to change is my fitness. But I have this idea that I hate exercise so I work myself into knots about doing it. So I am trying a ninja approach where I sneak up on myself with loads of tiny changes so I don't notice the big overall change. I used to throw myself at running or gym classes, get hurt or burn myself out. Right now though I have committed to doing. Something each day. Something as small as a 4 minute abs workout or as big as a lunch break walk. Nothing too big. Then when that feels routine and enjoyable I will add another step, like cutting out my mid morning snack. The key is to find out what the obstacles are and then find out ways over or around them, bit by bit.
The key is just to write about it and write consistently. Write down first thought that comes to your mind. Soon you will be able to organise them into a bigger groups and anger choosing the first fortress, put some small steps to concur it