Staying positive doesn't mean you have to be happy all the time. It means that even on the hard days you know that there are better ones coming.
Take a few minutes each day to count your blessings no matter how your day went. Even on tough days God provides us with many blessings – find them and thank Him for his grace. Use this uplifting time to visualize a wonderful blessing filled tomorrow.
This is the day that the Lord has made; let us rejoice and be glad in it.