Is it actually possible to choose what mood we are in? To a certain extent, yes. That was what I realised today. It worked for about 2 hours - choosing happiness and gratitude of course - until some kind of obscure anxiety set in. It was then that I realised the mind of helping others was a very powerful way of getting out of that self-centred negative mindset. Therefore, I focused on choosing the mood I was in - in relation to others. Whenever a negative thought came up about a person, I realised I could actually choose a different mood in relation to that person or situation. This included a thought about the person who bumped into me. And when I had a team meeting, I could choose to be positive about the team members. The meeting itself had a positive vibe about it the whole time.
That is not to say that such an approach is easy or will work straight away. Us humans are habitually negative. What you need as the basis is a way to discard that negativity. Practicing that first is key.