Unstable body image

One day I'll be devastated and in tears over how awful my body is. The next I'll feel quite attractive and content with it. Not in a manic way where I think I'm the hottest person ever but rather in a grounded healthy way, where I can acknowledge others are beautiful and but so am I, and genuinely believe it. Thing is, the bad days are 90% of the time and it feels impossible to feel better when I'm in one of those funks. My good body image days unfortunately are not born out of any healthy habits, they just happen to me. And yes I know sleep, hormones (especially being female, I don't get PMDD or mood swings but I definitely feel bloated or inexplicably uglier before/during my period sometimes), etc are factors at play, but emotional intelligence is not one of them lol. I have no emotional self-preservation; I'll impulsively mentally engage with things that will very likely make me feel even worse and my brain will be completely inflexible.

If I tried to tell myself in one of those funks "I'm beautiful too in my own way", I would think it was delusional; other women are genuinely beautiful, but any attraction anyone had to me would be from someone who's desperate, not picky, has a fetish, etc.. Because I'm not "truly" desirable. Every positive thought that came readily to me on my 'good' day will now be suddenly wrong or delusional. Like "holy shit was I really so dumb and narcissistic to believe that?" And even when I'm doing other things my mind will keep coming back to this. It'll have a chokehold on me. I kinda feel like I'm at the mercy of my mind and just have to wait for it to pass..

How is feeling better and staying that way so elusive to me when my brain is clearly capable of having healthy thoughts about my body?