Brain isnt letting me eat??

I’ve been wanting to gain weight for years now, and I’ve always been on the smaller side, even growing up. So weight gain is completely foreign to me. I’ve been taking some new multivitamins and ive felt a lot more hungry lately, but i can never eat my fill. Right now I know I’m still hungry but I just cant eat anymore. Anyone have any advice?? I’m gonna try to stick to smoothies to help.