Summarize:

Part of being mellow is knowing when you should actually change something that is gnawing at you. If you're irritated by a co-worker and do nothing to address it, then yeah, you are unlikely to feel mellow when you're at work. If your closet door is driving you crazy but you don't try to fix it, then you won't be feeling mellow for very long. The important thing is to approach the fixable problems in life with calm and resolve. Ask yourself about which things in life are causing you to not be mellow. Work to find a way to solve or address the problems that you are able to deal with. Along with changing the things you can change, in order to truly be mellow, you have to be able to learn to accept the things you cannot change. You can address a problematic co-worker to discuss issues you've been having, but you may not be able to change the fact that you hate the weather in the place where you live or that you have to live with siblings who pester you. Learn to recognize when a situation is out of your control and accept it with a calm mind. Let's say your new boss is driving you crazy, but you really love your job. If you try to resolve the issue and fail, then you have to learn to focus on the parts of your job that you love in spite of being annoyed by your boss. If you're the kind of person who doesn't know how to forgive and forget, then you are guaranteed to be less mellow. If one of your friends or family members has truly upset you, you have to work on being able to talk about it and to move past it, even if you haven't completely forgiven the person. If you walk around holding a grudge, then you are guaranteed to be in an angry, riled-up state instead of facing the day with calm and peace.  If you spend so much time obsessing over being mad at people who rejected you or fuming over people who have harmed you, then you will never be able to mellow out. Sure, it can help to talk about how someone has hurt you. But if you keep talking about it to everyone within earshot, you're only going to work yourself up into a frenzy. Keeping a journal can help you feel in touch with your thoughts and ready to face new challenges. Making a goal of writing in your journal at least a few times a week can help you check in with yourself and feel like you are in touch with your mental state. Doing so also helps you establish a healthy routine and gives you time to slow down and accept whatever the day has thrown at you. If you don't take a moment to breathe or relax as you write down your thoughts, then you will not be feeling mellow any time soon. Use your journal as a place for honesty and a reservation of judgment. Write down what you're thinking and feeling without fear or lies and you'll be on your way to feeling more at peace. A lot of people don't feel mellow because their wheels are always turning, trying to play out every move of life like it's a chess game. Let's say you're a writer who is deciding whether to be a librarian or to go to graduate school. Instead of planning out the next ten years of your life, wondering if you'll ever be able to publish a book, just do what feels right to you during that particular stage of your life. Focus on whatever you're doing right now and think about your next move without worrying about the next ten moves you'll have to make. If you learn to live in the present and to fully immerse yourself in what you're working on now, you'll be much more likely to succeed at what you're doing than if you're constantly wondering about where this step will lead you.
Empower yourself to change what you can change. Stop freaking out about the things you can't change. Don't hold a grudge. Keep a journal. Learn to take it one step at a time.