I think it has little to do with overthinking. My dad used to be really skinny. He'd get made fun of for being skinny despite eating tons of food. I was 135 pounds when I graduated high school and spent the next 10 years trying everything to gain weight. It wasn't until my late 20s where I started to gain some weight and got up to 170 at my peak but hovered around 165. Now that I'm almost 40, I managed to get to 195 (not really fat, because I go to the gym so it was well distributed but I had a bit of a belly going). I'm down to 185 now and have been trying to get to 175ish but it's a lot harder to lose weight now than it was when I was younger. I think that's just normal metabolism with age stuff.