I feel certain that I have SPD and I can relate VERY strongly to over 95% of your post and the other posts in this thread.
I hear what you are saying on relationships too. Knowing that you couldnt put up with anything less than a near-perfect match for you, and at the same time feeling that such a person does not deserve to be subjected to your cold emptiness. The dull sense of loss you feel after being around people with whom you think a relationship could possibly work.
I went out with a girl a few times. After a short period of time I became a bit of a prick toward her. Reverting to my usual self, trying to drive away the annoyance of social contact. I've felt bad about it, but when I met her again a good while later, I reacted in the same cold prickly manner.
I do this to some degree to everyone in my life. With the ones closest like my family I try much harder not to be too harsh, but without down time away from these people, I get more and more agitated, and shorter and shorter of temper with them.
Birthdays, Christmas, and gift giving/receiving are all right there too. Even good gifts or things I wanted make me uncomfortable.
At parties or get-togethers I rarely do more than sit and stare at a wall until whoever dragged me there takes my cues to leave. Or if I'm not with anyone, until I can sneak out. 21 is the drinking age where I am. I took my birthday off but still went into work when they called because they were short people - I had nothing else to do and no one I wanted to do it with.
I have one friend - who I suspect may have the same or similar personality type/disorder as me. And a very small handful of acquaintances with whom I will hang out but never call or talk to without being invited. Even then it's a chore. I often go weeks without contact with my friend or acquaintances.
Something I would have thought would have nothing to do with this, but maybe it does. I too play first person shooters online and am quite good. One of the best if not the best on the servers I frequent. I wont join a clan though. I wont even associate with the one all my local buddies/acquaintances set up. The same thing, nice at first, being non committal about it, then just ignoring, as if I didnt see the messages to me.
I too have things about myself that I dislike. I always thought that these things might be the source of my issues. Holding me back, keeping me inside my shell. I figured one day I would overcome these issues and be "normal." Well now I know better. There is no shell for me, only SPD. I havent been hiding from anything, and I've been suppressing far less than I had imagined. I always thought I couldnt cry because I suppressed my emotions too much. I no longer feel that this is the case. I havent been able to cry since I was a kid and cried about things like a scraped knee or getting yelled at by parents.
One thing that sticks out in my mind that makes sense now but previously made me feel like a bad person or that something was wrong with me was my aunts diagnosis with cancer and eventual death. Everyone in my family encouraged me to go visit her because she may not be around for much longer. I didnt want to because I knew I would feel nothing and have to fake caring about her. She passed away a short time later. I knew I should be at the funeral to give support to my family, especially my dad who lost his little sister. I couldnt sympathize with him or support him in any meaningful way. Not only that, but I stayed home from the funeral simply because it was my only chance to have the house to myself for 3 days. I used the excuse of not wanting to miss class so close to finals. It felt so selfish then, but I understand better now why I did it.
Praise makes me uncomfortable. I shrug it off like it was nothing. Same with criticism. I can take insults directly meant to rile me up and blow them off like werent even talking about me. Or I'll chuckle and say "whatever" and look at them as if they just made the most pointless and childish insult one could think of.
Like so many others here, I have struggled to find myself throughout my entire life. Never finding more than tiny shards of understanding amongst other people or through self reflection or minuscule bits of writing that you can truly relate to. It's a little mind blowing to stumble onto this forum and read the posts, feeling an instant bond with the members here, as if nearly every post could have been written by your own hand. Knowing exactly what is being conveyed by each and every person, having lived the experience a thousand times over.