Remove words per year in a corpus

I am working with a corpus with speeches spanning several years (aggregated to person-year level). I want to remove words that occur less than 4 times in a year (not remove it for the whole corpus, but only for the year in which it does not meet the threshold).

I have tried the following:

DT$text <- ifelse(grepl("1998", DT$session), mgsub(DT$text, words_remove_1998, ""), DT$text)

and 

DT$text <- ifelse(grepl("1998", DT$session), str_remove_all(DT$text, words_remove_1998), DT$text)

and 

DT$text <- ifelse(grepl("1998", DT$session), removeWords(DT$text, words_remove_1998), DT$text)

and

DT$text <- ifelse(grepl("1998", DT$session), drop_element(DT$text, words_remove_1998), DT$text)

However, none seem to work. Mgsub just substitutes the whole speech with "" for 1998, whilst the other options give error messages. The reason that removeWords does not work is that my words_remove_1998 vector is too large. I have tried to split the word vector and loop over the words (see code below), but R does not appear to like this (running forever).

group <- 100
n <- length(words_remove_1998)
r <- rep(1:ceiling(n/group),each=group)[1:n]
d <- split(words_remove_1998,r)

for (i in 1:length(d)) {
  DT$text <- ifelse(grepl("1998", DT$session), removeWords(DT$text, c(paste(d[[i]]))), DT$text)
}

Any suggestions for how to solve this?

Thank you for your help!