吃means"eat＂, 不is a negative word， means＂can't＂ . 吃不下is more likely the words" can't tuck in " 吃 is a verb. "tuck" ,下 is a prep."in"(a direction prep. mean into the tummy) The things make me feel upset/anxious and lost appetite.

吃饭 is a verbal phrase, 不下 is the result of the action 吃饭，that is, the action 吃饭 can not be completed or finished due to worries.

I think basically it means “I have no appetite “

吃不下means I can't eat or I don't want to eat 饭 means food 吃不下饭 means I can't or I don't want to eat anything.

吃不下is together. 吃得下 吃不下 is a way to describe the possibility, for I could eat or I could not eat.吃得完 吃不完 for being able to eat all or not being able to eat all, just partly