What we often neglect when looking towards the future is what we are doing now. We only see the positives: how that dream internship will look on a day-to-day basis and how it will boost your experience for grad school, how writing for the paper will give your writing more exposure, and more. However, I urge you to look at the schedule you have now. Can you meet the time commitments? If not, how will you do so? Do you honestly think you can manage all you are doing and the duties of this new opportunity?
If you can, great! Some people are really good at multitasking and that is a beautiful thing. If not, look at your schedule. What classes do you have? How much homework do you have? What do you do outside of school? Can you drop anything? If your schedule is full to the brim, the importance of saying no, whether it is to the new opportunity or a continued obligation, is necessary.
If you want to discontinue the new obligation, you most certainly can without feeling guilty. Acknowledge that you have had a good experience but that your time commitments do not allow you to continue and, if you want to leave the door open to return, that you would love to return if your time commitments allow you to do so. If you are turning down the new opportunity, do the same thing. Express your gratitude for being selected, but that you look forward to applying again when your time commitments allow you to do so.
In school, we were taught to say no to drugs. In college, we also learn to say no to burning out.
-- Ricardo Canelo