I've kept a journal since 1964 (I was 12, inspired by Anne Frank). It has served me well. It helps me process emotions, and gives me a reality check.
When you write, I would suggest having no "rules"--just write whatever spills out of your head or your heart. Over time, you'll develop a style, or a rhythm, and it will become easier. Also, don't do it for anyone else. Don't imagine that someone will read it and benefit--that will keep you from being totally honest. Just do it for you.
I have found that I crave writing at certain times of my life. I can't say it solves problems. One of the things I wrote in my diary in 1974 was "I can't continue a relationship with 'A' because he's an alcoholic." Hello.. here I am 43 years later, in a relationship with the same guy, and I probably have the same emotional problems.
But don't let that stop you, because it really does release the emotional pressure valve. No matter what, you learn about yourself, and you have a completely unbiased, non-judgemental audience. And that's.. nobody but you.