For me, I didn't have a lot of self esteem for a long time because I downplayed and minimized all the fucked up shit I went through, for a number of reasons, and it took a lot of radical acceptance and REBT to accept and acknowledge that yeah, I exhibited a ton of strength and resilience and bravery just to even be alive today. And by admitting that to myself and trusting myself and giving myself that understanding/acceptance/love/whatever, I now care way way less about how other people feel or think about me, because they don't know the first thing about me. That may or may not be a factor for you, I don't know how easy or hard your life has been, but I'm mentioning it anyway.
This.
Not trying to be rude, but yeah some of the advice on the thread is a bit ill-given in my opinion.
I tried to do the conscientiousness thing forever. Combed my hear, smelled nice, made conversation with people, lost weight.
Truth is, nobody really gives a shit about that. You can be a fat dude who talks about conspiracy theories all day at work and people will still be cool with you. Don't be Patrick Bateman. People don't give a fuck.
I think the honest truth is if you didn't have self-confidence instilled in you young, then it's just something you're not going to naturally have as much as some others. It's not the end of the world. But you have to stop looking around and panicking, wondering why you don't "have it".
The biggest problem with modern self-help/life advice is it's rather gaslight-y in nature. If you feel like a failure and want to have self-respect, you don't endlessly scrutinize yourself, looksmaxx, and work every second of the day on every aspect of yourself until finally you're an "acceptable" person. That shits tiring and will make people around you on edge because they can see the tension you put off of trying to be a perfect person. There's nothing wrong with improving as a person but the very first thing you have to do in order to actually feel any satisfaction in the results is to respect yourself a little first.
Care about what people think to the extent it allows you to live a comfortable life (i.e lets you keep your job, doesn't harm your personal life). Otherwise, consider the source when being criticized.
The only really two things that have helped me is sleeping well and exercise. It's not even because you lose weight, people just treat you different after getting out from a good workout.
Try getting up three hours before work. Preferably early in the day.
Don't spend a lot of time online as a lot of it's very cynical and meant to make people overly introspective.
Listen to people more than you talk about yourself.
And just get to a point where you're not overthinking everything. That's impossible for me but it might not be for you.
The best self-improvement happens when you're not even thinking about it.