Choosing happiness requires being aware of your emotions and your thoughts. There is always a way to ratchet your happiness factor up a notch.
Use a 0-10 scale and evaluate how happy you are in this moment.
Now grab a thought - it can be a memory, it can be something you observe in your present moment, or it can be a visualization of your future. Fill yourself up with that thought and take your happiness level up a notch. You can do it!