You're a SAHM mom of 2, full-time stu, writer, and pol activitist and wonder how you can fit it all in? I wonder how you did it BEFORE the 2nd baby!!
My advice (having done the single mom, working full-time, full time stu, highly social, highly involved thing) is prioritize your life. What is the most important thing to you? If it were me, it would have to be a choice bw the mom thing and the student thing. Having enrolled in and paid for (im assuming) your last year of college (whether it be undergrad or grad school), and it being ONLY a year, that would have to be my first priority. Now's definitely the best time to complete your degree before the babies get any older (start school, etc), but you can only do that if you're serious and if you have help. Meaning, your husband would have to be very understanding and just as dedicated to your degree as you are. He will have to take over the parenting as much as possible and you should let him.
Also, I understand that writing and politics may be important to you, but how important? I would dump those activities as soon as possible to focus on what's most important: your degree and family. You sound overcommitted...if so, decide what's sapping most of your energy, call the person in charge, and apologize for leaving them in a lurch, but explain that you simply cannot fulfill the responsibility as takes too much time and attention away from those things most important in your life, that more than likely its only for a year, and you hope to be able to work with them in the future. If there are two or three acitivities that keep you sane (ie, provide adult conversation outside the home, social interaction), make an effort to take of yourself and sanity in that way. But disengage yourself from anything and everything that doesn't support your #1 priority, whatever that might be.
Being a mom, wife, housekeeper, etc is a full-time job in and of itself. Being a full-time student requires nearly the same amount of time and attention if done correctly (which is why we are encouraged to complete an education prior to marriage and children - not that that stopped me either). You can't do everything. Despite what anyone tells you, one cannot be everything to everyone, be fullfilled and have it all without becoming overwhelmed, incredibly stressed, and ultimately depressed. We weren't made that way.
Good luck!