Ultimately, each of us is responsible for our own happiness. Sometimes it's easy to be happy, but the challenges of life often make happiness seem elusive. One way to keep going even when things are difficult is to think positively. Mantras can help. Here are 10 phrases to motivate and lift you when you're feeling down.

1. Life does not have to be perfect to be wonderful. This mantra inspires you to find wonder and joy in an imperfect world. Joy can be found in simple things, like a beautiful fall leaf, a child's hug, a text from a friend or accomplishing something. Look for something wonderful in your life.

2. It's a good day to have a good day. I like the straight forward nature of this saying. Why not have a good day? Try to start the day with a positive attitude and see what happens.

3. Do more of what makes you happy. Even a day full of work and tasks should have some happiness in it. Take a few minutes each day to do some things that make you happy. Hug a child, dance to your favorite song, eat something delicious, take a few minutes to call a friend or make time for a hobby you enjoy. Life is full of unpleasant tasks. Counter those with things that make you happy.

4. Let it go. Ever since the movie "Frozen" became popular, this mantra has been one of my favorite. It's saved my mood on many days. Now, if something is out of my control I let it go instead of feeling frustrated. Letting go frees up emotional and mental energy for things you can control.

5. I am enough. When I was in elementary school, I learned the phrase "I am lovable and capable." This mantra teaches the same concepts in a less new-age way. If I'm feeling less than adequate about myself, "I am enough" is a good reminder that I am fine the way I am, that the basic me is pretty special.

6. Bring your own sunshine. I strongly believe that we have to make our own happiness. I love the imagery of this mantra, which makes me picture a lovely day full of happiness and peace. If everyone around you is grumpy or negative, you might have to bring your own sunshine. It's a good idea to bring enough to share, too.

7. Attitude is a little thing that makes a big difference. (Attributed to Winston Churchill) I've been teaching my daughter about sports psychology lately. She's running cross country and is often negative about her upcoming races. I've encouraged her to start thinking positively about the race and her personal time a few days in advance. The weeks she's done this, she's raced better. This example is one of many that shows how important attitude is. If you find yourself with a bad attitude, try to change it. Click here for instant bad mood busters.

8. Every day might not be good, but there is something good in every day.At the dinner table, we often take turns sharing our "highs and lows" from the day. This practice helps the children realize what's good in their lives and how we are blessed as a family. Invariably, my youngest daughter continues to think of and share her "highs" throughout the meal. Find something good in every day and you'll feel uplifted.

9. I choose to be happy. This mantra is simple, yet effective. Choose to be happy. However, if you feel like you just can't be happy, consider seeking professional help. Sometimes clinical depression sets in, making it physiologically impossible to come out of a funk.

10. Realize how blessed you are. An attitude of thankfulness will often bring about happiness. Keep a gratitude journal or make a list of 20 blessings in your life. Then, take some time to think about the people who bless your life. Even if you're not a religious person, realizing the ways you are blessed will make you feel happier.

The next time you're feeling depressed, inadequate or grumpy, try using one of these 10 powerful phrases to change your mood and outlook. Or, find your own mantra and see how the power of positive thinking can help you find happiness.

Amy M. Peterson, a former high school English teacher, currently lives in Oregon with her husband and four children. She spends her days writing, reading, exercising and trying to get her family to eat more vegetables.