How can I remove myself from a project I'm unable to complete? I'm in my first lab and while working on my main project the opportunity came up to help another senior researcher with theirs. It seemed like simple tasks at first, but I'm now convinced that this will take way too long to figure out and it is taking significant time away from my main project. How do I proceed?

TL;DR: Bit off more than I can chew, how do I amicably inform my supervisors without seeming too incompetent?
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.