Does this sentence sound right to native speakers? English learner seeks help?

It doesn't sound right. The verb "to prejudice" is a transitive verb, indeed, but when the direct object is the same with the subject of the sentence, things don't sound normal. Here's another example "I used to set myself on my enemies." Both your sentence and the one used as an example are possible and correctly formed, but not natural and not native-like. On the other hand, poets who write in their native tongue do to their language what a non-native might do without intention. Things are more complicated, that is why I will stop here and fix your sentence: "I used to have preconceived ideas about those who celebrate foreign festivals." (also: I used to judge those who.../ I used to look down on those who.../ I used to disapprove those who...)