Shift to focus on what is okay rather than what is not okay. Try this in each moment.
Imagine being at a salad bar (meeting, party, movie, golf course, traffic jam etc). You have a number of choices for building your own salad. You have your preferences and you can choose from what is available that meets your preferences. You cannot choose what is not there.
When you review your choices and select what you prefer from what is available in each moment, you
start to live the life that you want. You may need to momentarily let go of the fantasy reality that is not currently available but you will be much happier in the mean time.
At times during the day our mood may shift from neutral to positive which we usually enjoy. If we shift to negative we start to feel stress and we would prefer to get back to neutral or positive. The sequence is usually from negative to positive and then back to neutral. This is like a diminishing sine wave that settles back to a nice even neutral position.
Actually a positive mood although it is sought after is actually a shift away from the homeostasis of the body mind. Rest in neutral, enjoy the positive when it comes up and use a selection of preferences from what is actually available to rise up from negative moods.
Experiment with this:
Over the next four hours look for moments when you realize or become aware that you are not happy. See if you are focused on what you perceive is lacking and the sense that if that missing item were there you would be happy.
Immediately drop the wanting of the missing item that you brain has filled in and find something to be happy about and satisfied with. Find something that is right with the salad rather than what is wrong. Of all the choices available which one do you prefer and then decide to choose that one.
Start living by choosing your preferences rather than what is lacking or unavailable. When you choose the yummy donut rather than the hole you end up with something rather than nothing.