Man, is it difficult to stay positive, even when some days are horrible. About a year ago, I decided that I would start a list on my phone of positive things. Some of these are pretty big (like finally paying off our car!) while others are small, like simply buying a new nail polish. This is my version of a gratitude journal.
Some days, I might have ten positive things to jot down. Sometimes I'll go a few days without writing down anything. But it's become a habit now, something as natural as getting up in the morning. This list is also an excellent tool for when I feel anxious. I'll sit down, read the list, and by the time I'm done, my blood pressure is lower. It sounds simple, and maybe even stupid to the cynical sort, but counting the things you're grateful for can make a big difference in your life. We tend to only remember the negative, and the good things have a harder time sticking in our brains.
I never used to really dwell on the good things, so I should know. I was the person who would rage when the train was late, or over some stupid inconvenience. I mean, I'm not some sort of cheerful robot. I still do get teed off by things, and sometimes in a major way. I still suffer from anxiety. But I try to keep it positive, and think "will this even matter in ten minutes?" And if the answer is no, I try to let it go.
I also don't beat myself up over the things I can't control. Sometimes anxiety will come out of nowhere, and instead of letting it crash over me, I will take a minute to think about my personal positives, or read a quote from the positive quotations jar I keep on my desk. It doesn't always work, but the fact that these tactics work at all is a huge thing for me.
I still complain, I still worry. But now I know that it doesn't really change things, and if I can, I try to choose a positive thought.