Thanks for your thoughtful post. I have used journaling for a break up as well.
I also use it in my current relationship. I express my feelings and things that make me angry in my journal. Getting these feelings out help me to not take them out on the person I love. It also helps to see some of my issues written down. When I look back I can make a judgement on the ones that are important that may need discussing and the ones that are plain stupid and not worth thinking about.