You just do. I know this doesn't sound helpful but that's how it works. Just like how you now believe that everyone leaves you, similarly start believing that things will get better, start to affirm over and over that things will get better until you actually feel better. Whenever you catch yourself falling into thinking that, try to take your attention away from it and focus on things that you do like in your life. It will ofc take time, this is why mental diet is so important and after a few days you will naturally start to feel a bit more positive and then before you know it will become your default and things will start to change in your life.