A lot of it is fake it until you make it in my experience.
Start on the biggish stuff like I'm a good person, I wouldn't deliberately hurt anyone Iv tried to be the best at my job parenting etc I can
Then look at how other people seem to see you as in
If Jane,john,Fred,Louise, (made up names) who is a woman/man I respect likes me or like then there must be a reason and even if I don't see it in myself it would disrespectful to her/him to ignore the fact she/him does see something in me.
Then look at the body image stuff and choose even tiny things that you like/can tolerate about self I have good legs arms teeth it could even be I like my nails whatever.
If you start small and steady and be relentless about keeping it up while at the same time ripping apart the negative impressions (even when that's all you can see) eventually some of the positive will start to stick.
It also helps about being honest about the things that may not be the best like I can be grumpy or I should look after myself hair eating etc better. So then it becomes a more overall picture. Rather than just I feel good about myself it's about what do I have to do to be the best person I can and what have a already got or achieved