Meaning of 只 in 只把思念积压在我心头 Ok so firstly I take the sentence as a whole to mean.

The missing is accumulating in my heart (i.e. I'm missing you more and more).

Provided I've got that correct, what function does 只 perform here. Is it superfluous and just added as emphasis?

This is from the lyrics of 思想

If it's at all relevant the preceding line is

为何你一去便无消息 

只把思念积压在我心头
Here, I thinks 只 is short for 只能, so it actually means since you leave without a message, 我只能把思念压在我心头.