I've found that I spend a lot of time imagining myself as someone looking at me and disapproving.

Ah! Thank you.. I find that describes precisely what's most wrong (currently) with my own sense of self. Earlier I would go days thinking about something that I said/did which probably nobody else noticed and feeling bad about it. One way I've recently started managing it is by actually forcing myself to notice similar mistakes made by people I like and respect and realizing that nobody thinks less of them for those slip ups.