It’s a good goal to tell yourself “I want to be happy in this moment” because the desire often precedes the feeling, and must, especially when you are struggling. However, it’s not always possible, or even appropriate, to be happy all the time; and if you persist in trying to make yourself happy when the circumstances are pushing things the other way, then you risk feeling as if you’re failing, which just makes everything worse.
A greater goal, one that is both more broad and deep than saying “I want to be happy in this moment” is to tell yourself “I am invulnerable in this moment”, because that means whatever is going on, whatever the conditions and circumstances are in your life, they will not break you. You can get through them. And when you do, you will find it easier to embrace those feelings of happiness that exist for us all.
I do believe that happiness can be chosen; but I also know that sometimes, it seems too difficult to choose in a particular moment. But invulnerability is about strength, not happiness — and if you can find and hold to the strength within in those moments when you aren’t happy, you can connect better to those moments when you are. And you will be.