Obviously, you can see how mattering, especially if it is a durable conviction, would support emotional health. Lots of data show this association. But it should also be obvious that mattering is hanging in metaphysical thin air. Why, exactly, do you matter? And if you don't feel like you matter, how can you come to believe it?
Left to ourselves, mattering is hard to come by. Telling yourself you matter will only get you so far. Mattering has to be mediated, has to come to us externally, from others. That is the point of the Dietrich Bonhoeffer's quote that I've shared many times before:
Help must come from the outside...God has willed that we should seek and find God’s living Word in the testimony of other Christians, in the mouths of human beings. Therefore, Christians need other Christians who speak God’s Word to them. They need them again and again when they become uncertain and disheartened because, living by their own resources, they cannot help themselves without cheating themselves out of the truth...The Christ in their own hearts is weaker than the Christ in the word of other Christians. Their own hearts are uncertain; those of their brothers and sisters are sure.
We mediate mattering for each other. When our hearts are uncertain we speak life into each other. Help comes to us from the outside. Mattering is spoken into us. In our homes, with our friends, in our churches. Mattering should not be reduced to solo self-talk, for it most powerfully resides in relationships of care and love, relationships that convince us that we matter.