I truly understand your conflict. But you must take a step back and look at your own future, and protect what is important. I imagine, and hope, your answer is your school.
I was in your exact shoes, except that I would never volunteer, EVER, to be a treasurer, I was just the secretary and responsible for the monthly newsletter in our moms group, plus notes, etc.. I felt duped by the board when the duties were described to me when I agreed to the position, because once I was in, turns out, they wanted the newsletter done an entire week before the prior secretary told me, and I literally could not do that. It completely conflicted with my other personal obligations at that time. And I also felt the wrath of these board members who were upset as they had their own schedules to maintain for review and turn around back to me.
Fast forward, we ended up moving out of state and I never saw them again. But even if I did stay in the area, I realized I would not have maintained friendships with any of these woman who were disappointed in my output. Gosh, we're all moms and they were rather rude about the scenario.
Look, ES, do not for a moment beat yourself up over this. Things change all the time. It's inevitable. Finding and keeping a treasurer for any organization is difficult, as they have the most work of anyone on a board.
Tell them as soon as possible that your predictions came true, that you have this grant funded program on your plate now, (big YEAH!!!) and that to honor them you have already requested a one month extension on your program, in order to correct the erroneously deleted bank statements, and this means they have one month to advertise and find a replacement treasurer for the remaining of the year, as this program takes precedence in your life right now.
Tell them you are sorry for any inconvenience it causes them, but you cannot juggle this position with your other existing responsibilities. Keep it plain and simple and do not allow them to coerce you to stay on.
Now, you might offer a small amount of time to transfer the materials to the new treasurer.
Also, what about asking for immediate help. Can't any of the other board members help you input bank statements?