Rather than being insecure about a body part, or thinking I'm too fat/thin etc., I'm self-conscious about my posture, gait and body language. If I'm not doing anything with my hands, I don't know what to do with them, and I'm very self-conscious about having these two limbs sticking out of my body that I'm awkwardly trying to look like I'm casually not thinking about :S I end up having my hands clasped together, in my pockets, or arms crossed a lot of the time, because just hanging by my side feels unnatural. I like having food or a drink in my hands when socialising, so that I can stop fretting over what my hands are/n't doing.
I used to have pretty severe body image issues, which in all seriousness probably would have met the diagnostic criteria for body dysmorphic disorder if I saw a pscyhologist. I was constantly bullied for being skinny as a child (normal people don't do that to me anymore, but lifters have a tendency to still be right diks; to this day, if anyone in the gym makes a jab at me for being <100kg, depending on my mood in the moment it takes either a firm grasp on the doctrine of grace, or just a desire to avoid negative social/legal repercussions, not to turn around and weaponise the nearest weight), which, among other factors, taught me to look for everything wrong in the mirror. It turns out that when you ask the mirror what's wrong with you, the mirror doesn't hold back -- if you're looking for something wrong with your body, you will find it, so it's quite irrelevant how "good" you look if you're looking for whatever isn't "good." When I was a teenager and started weight training, I got visible results quite quickly. Because I was so skinny starting out, even tiny amounts of muscle were a significant difference. I went from about 45kg to 58kg in 12 months (in hindsight, that's dramatic; the whole "gain 1lb/wk" crap pushed by the interwebz makes 10-15kg in a year sound pathetic when it's actually right on track), and I got a lot of positive comments from that. Had I been looking for the positive, my self-esteem would have been through the roof, or at least better than before. But I wasn't looking for the positive, I was looking for things to fix. And as I got more focused on strength training and bodybuilding in general, the result was to become hypercritical of everything that differentiated me from some ideal. Rather than becoming more confident with my body, my body confidence decreased. The stronger I got, the weaker I felt; the more defined my muscles got, the fatter I felt; the bigger my muscles got, the more painful it became that they weren't "big enough."
It took about 6 months of forced leave from the gym for me to break that cycle so that I could exercise and not have that mindset. It took a further 4 years before I could focus training on anything remotely aesthetic in purpose without it rapidly becoming a slippery slope into that mindset.