Writing your thoughts and emotions down is a great idea. I know it helps me, but I too have the problem of fingerpointing one issue that would 'cause my depression' I am a recovering alcoholic. I belive that my depression came before my drinking, I just didn't know how to deal it. But there was no clear moment that I said 'I'm depressed' I just know that I am. I am on meds and have been for quite a few years.
I always incouraged my children to write, it helps you to see what's going on with you. But it is best to run it by someone so they can give their view on it too. I know I am critical of myself and that has always been an issue.
Trust me being a parent, you want to know when your kid is hurting. I know it's tough and you can't predicate the outcome, but you will know when the time is right. Don't wait till they see you lying in a hospital room. Give them a chance.