is this an example of antithesis? I think it is because it has two contrasting ideas, but I am not 100% sure...

"The problem isn't that we're eating the wrong things at these restaurants; the problem is that we are eating there at all."

Yes, the sentence you provided is indeed an example of antithesis. Antithesis is a literary device that involves the juxtaposition of two contrasting ideas in a balanced or parallel structure. In this sentence, we can see the contrasting ideas being presented: "eating the wrong things" and "eating there at all." These ideas are opposing each other, highlighting the negative connotation of eating at these restaurants.