*hugs*What is a man but a miserable pile of insecurities?
With that in mind, a comprehensive answer would have me typing here for weeks, so I'll narrow it down. The root of my insecurities is my own inferiority, specifically with regards to relationships. I don't like to admit it but I am the sort who seems to require frequent - e.g. weekly or even daily - reminders of my worth and the strength of my friendships, lest I feel that things are slipping. Granted the reminders aren't much, all I need is a friendly message or something similar, but without them even the strongest friendships falter as I recede into my deep dark tunnel of brooding and self-criticism. Romantic relationships are even more unstable. I simply don't feel like I could ever actually be important to someone, and given how passionately I feel about those I hold dear, I have extreme difficulty accepting that anyone could ever feel as strongly for me as I do for them.
Physically I'm a wandering bundle of insecurities, but as I can't do anything about those, I try not to dwell on them.