I've learned over time that even in the midst of pain, there is always, always, at least one thing to be grateful for. Even if it is as simple as the fact you have shoes on your feet. Or a job. Or a friend. It may not mean that your life is exactly where you want it to be right now, but it's a start.
Focus on it. Focus on that one thing. Thank your God for it. Appreciate it. Take care of that one thing. Treasure it.
You will be amazed at how much your attitude can change when you choose to focus on the good things in your life instead of dwelling on the bad.
If you really want to be amazed, try giving something! Even if you think you have nothing to give, you do. Give your time to someone who needs a listening ear. Give a smile to someone who looks like they could use some cheer. Give a hug to someone who is in desperate need of love.
Not only can you choose to be happy, you can choose to share that happiness with others by giving it away. You are quite powerful! You just need to believe it.