The forums have been a bit inactive lately, and I'm sorry, but I don't always know the answers.
In this case, I struggle with the same thing. One thing that has helped me somewhat was recommended by a therapist of mine. Take a piece of paper, and write a list of good things about yourself. Aim for five at the minimum, even if you have to write something simple like "I have a nice smile." or "I brush my teeth every day." You can also try writing a list of good things you have done in the past week.
It helps me stay more positive.
Another practice comes from my studies in Buddhism. A useful slogan is "When you find yourself stuck, do the opposite." So, if you are having trouble feeling like you don't belong, let the thought dissolve, and then think to yourself "I do belong. My friends and family care about me, and would be sad if I were gone."
One more thing is to practice something called tonglen. When you're covered in this sort of fog, inhale slowly, thinking of all the other people in the world who feel left out or misplaced. Then exhale slowly, breathing out feelings of happiness and belonging. It should help you to identify that you are not alone. Keep inhaling and exhaling, keeping each breath even with the other, and you should start to feel a bit better.
This can be used for other negative circumstances as well. Breathe in the negative feelings, and then breathe out whatever emotions would relieve the negative feelings.
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