Wow. What a great topic. I currently type as sx-dom precisely for the issues I have with this.
I'll start with the truth: I got put down by my peers a lot as a kid. They told me I was ugly and disgusting and weird. For years. And, I'll be upfront about it...that changed my self-perception, permanently. I wasn't mommy's pretty little girl anymore, I became ugly, disgusting, undesirable. Being a 4w5, that's pretty much become a source of identity for me--one of the ways I'm weird and worse than everyone else. I can't change my thoughts on this matter, even when I want to. I just feel ugly, disgusting, creepy, undesirable, etc.
And, it's brought out the worst of me. I'm aggressive toward attractive members of my same sex; I spend hours in self-torment about how much I physically suck; the whole "Let's remind ourselves why no one will ever like you..." lecture. It's like my entire identity is pegged around this one hang-up.
THIS WAS ESPECIALLY BAD IN COLLEGE. All the more so because guys never seemed interested in me. Just further proof of how ugly I was, right? I developed entire self-esteem issues solely around this concern. Like the poster above says, I feel less human. Really and truly. Pathetic (and up till now, a highly guarded secret!) but true.
Anyway, I can't go around whining about it forever (actually, I've never been able to talk about this to anyone), so mostly, I've just been highly rejecting. "Who needs that crap?" "Oh, you're worried about your physical appearance, are you? You vain little batshit." "I don't need ANYONE. I'm not DESPERATE." That sort of thing. Everyone thinks I'm asexual IRL because I've taken such a cold stance towards these issues. I typed as sx-last for a long time till I realized I've just got a neurotic anti-manifestation of it in terms of desirability.
I've never even been able to articulate this to myself till I found out about it, actually. I doubt I could say it out loud, but telling random strangers on an internet forum is ok, right?
So, to the OP, I can sympathise anyway. It does take a toll on your mental health, as pathetic as that sounds for me to say that. Good luck!