I have found the only way I can not be obese (and I mean the lowest BMI I have had is 24) is the most extreme diets - keto and now carnivore since keto is too hard to stick to
Though I don’t envy you. You have no clear signal that you’re eating unhealthily, but suffer the same ill health that us fatties do if you do eat unhealthily
You need to pump iron and eat protein to gain healthy weight