The same thing started with me around 14-15 and just got worse. I slowly lost friends and could not make new friends due to mental illness. You know mental health professionals will tell you that getting involved with others is the key if you put forth the effort but adulthood is like a fraternity and there is peer pressure and expectations in every walk of life and if you don't meet them you will certainly face rejection. Some people are not cut out to live with (or marry) others and lots of people are forced to live alone as a consequence.