Whatever you dwell upon in your mind, you give power to. So of course it makes sense to focus on what you want rather than what you don't want. Yet there are many ways in which the things you don't want can sneak into your thinking.
Worry and doubt focus your thinking on what you don't want. Rather than worrying about the bad things that might happen, direct your actions toward making positive things happen.
Complaining can also get your mind sidetracked into thinking about what you don't want. What do you complain about? The things you don't like. Rather than complaining, take action. Action moves you toward what you do want.
Anger is one more way to get your thinking negatively directed. Rather than getting angry about what you don't like, use that energy to give you determination for what you want to achieve.
Keep your mind positively focused on the good things that life can offer. Those things will grow stronger and more abundant in your world.