This is a good question and everyone's answer is different probably. For me, I had the resources and willpower to find a good therapist and I went in with the mentality of "this person is going to help me figure myself out so I can appreciate myself more". Going to therapy is one thing, but going in with the mentality of knowing what you want to get out of it is really the best way to see results. There's something different about having a professional person, who you trust, telling you "actually, it's okay to do X." I personally never would've taken friends or family seriously because I was too in my own head about them telling me what I want to hear, and for some reason paying money for that made my therapists advice more legitimate.

Beyond that, self-esteem has the answer in the name. It should come from you, not other people (with the exception of my therapist, who felt like a trusted second opinion). I basically used my therapist to bootstrap my way into understanding that it was harmful to constantly shit talk myself and self-deprecate. So I started turning things around. I used to think it was cringy to think highly of myself but when I finally gave it a try and said to myself "wow, I just took care of myself!" after I tried exercising for the first time in a decade+, I felt like I could actually believe what I was saying. And things kind of started to snowball from there.

Now one other big caveat was me realizing I was trans (turns out heavy denial leads to self-esteem issues, wow), but again, that's where a good therapist can really do wonders.