How can I remove myself from a project I'm unable to complete?
I suggest that you just tell your supervisor and the other researcher that you are finding it difficult to give the other project adequate time and effort given your main and more important tasks. 

It is fair to say that you underestimated the effort involved, but then, others probably did the same. Treat it as a problem to be solved. 

You don't need to actually remove yourself or make a demand, but just state the facts as they are and ask for advice. Perhaps the advice will be that you drop it or otherwise refuse your effort on it. 

But you also don't need to accept conditions that are unreasonable, so make sure the conversation ends in some consensus. Then, you only need to "upset" people as a last resort.