I don't think we can directly choose our feelings. But we can make choices that result in our feelings changing.
What we have direct control over is our attention. Many things are happening at once, all the time. What will we focus on?And how we will respond to that.
We can generate happiness quite quickly by focusing on something good, and choosing to feel grateful for it. This is a great way to get rid of the blahs.
We can generate happiness by allowing beauty and harmony (nature or art) to relax our entire nervous system.
We can generate happiness by taking something neutral, and making it positive. A Zen master teaches, Last time you had a toothache, didn't you say, "If only I didn't have this toothache, I'd be happy." Well, to day, you don't have a toothache. Keep that promise to yourself. Be happy for your "non-toothache."
We create a lot of misery by having specific expectations, such as wanting a holiday event to go a certain way. We create even more by allowing other people's ideas, such as "the holidays should be happy" to affect us.
When we must face something difficult, let us do it in a clear, helpful way. If we can't be joyful, we can, nonetheless, be peaceful.
For there is a deeper Peace that lies in our consciousness, completely independent of all circumstances and moods, just waiting to break forth into the world.