Hmm self-image is hard. Sometimes I feel completely confident but then sometimes, often when I've messed up on something or not accomplished something I wanted to or are remembering something embarrassing from the past, I feel like I'm a horrible, worthless person. I can usually rationalize myself out of it, like I have this underlying idea that I'm a pretty competent person, and I just have to remind myself that everyone is flawed, nobody's perfect and nobody can be, and then I'll be alright. The thing is, if I try to think about it, I can come up with a list of my achievements that make me special, but the feeling is deeper than that. I can just as easily refute my list of accomplishments, reminding myself that everyone else is also just as special and important as me (maybe even a little more, a part of me whispers) and loose track of that deeper feeling, then I'll start to feel bad about being "proud" or "vain" or something and so on and so on. Sometimes, and I don't know what causes this to happen exactly, but I can suddenly, consciously just decide to be confident. Unfortunately, that only works if I have the energy and the inclination to do so.
As for what anyone else can do about it... If someone complements me, and I can tell they really mean it, I am rather surprised but also quite pleased. My confidence lifts unconsciously, but if I start to think about it too hard, I get stuck in overthinking it and "what if they were wrong" mode. If someone I care about, whose opinion I trust, says something nice about me, especially along the lines of "I'm glad we're friends," it makes me amazingly happy. Like warm balloon welling up from my stomach to fill my chest, instant smile, wordlessly happy. But that doesn't mean that after they have left I won't go back to anxiously analyzing myself and wondering what on earth it is that they like about me.