yes I can relate to this completely. My SAD and depression broke in quickly when I turned 18 and started university in 2006.
Suddenly I couldn't and didn't feel like myself even around my closest friends anymore. Even around my own family to a degree.
Took a few years before things got bad enough but I eventually withdrew socially and stopped talking to my friends altogether for awhile. MY gf the one person I still felt normal around finally broke up after 4 years (2010) with me so I FINALLY made the determinded effort to go get help.
Have done medications CBT books and CBT groups. Things got better for awhile, Ive been able to function to a point where I feel comfortable that my anxiety is in my control and not noticed too much by others, went to class got a pt time job.
Then I went off antidepressant thing spring due to side effects and relapsed pretty much completely. Ditched the job I had this spring, and have been cutting myself off from my friends again the past few weeks.
There are basiclaly two levels of it for me, but SAD and depression are with you wherever you go. Same with most mental disorders. Because of the difference in the way you feel and think, you often end up feeling isolated from people around, regardless of how far you physically are from other people. It's the differences in your perceptions, in thinking in emotions and behavior that creates an isolation as compared to the mentally healthy people around you. Also comparing yourself to others when you go out can make u feel worse also.
level 1 - physical isolation: anxiety to out of control don't feel safe or in control enough to be able to see a friend and carry on a conversation without my problems coming across and being like wtf k and potentially losing a friend or damaging respect or the relationship
level 2 - mental isolation: on medication and feel good with progress I've made, can hold a part time job or take a class, go out and hang out with friends without even thinking about it/debating about not to, just saying screw it I got this. Go and do these things and can hold my own for the most part but am FAR from being able to ENJOY myself cause although anxiety is more under control, is still very present and am not actually being myself or having fun so much as I am still hiding or covering up my issues.