Hi R.,
While I'm not a stay at home Mom, I do own my own business, work full-time at another and have an almost 2 year old son. I am not totally in your shoes, but still must balance work/life/husband. I am in a mastermind group and we just recently had this discussion because it is a common issue.
You have received some great advice (I haven't read everything) about getting organized. I especially like the post from Patty M. Remember to take baby steps. The thought having to do everything will shut you down and you won't get anything done. Every night make organized lists. I don't know what your business is, so I am giving general advice.
Get a timer and spend 15-30 minutes each morning, afternoon and evening reading and responding to emails (no more than that timer allows)
Use the timer method for each list item and focus 100% on it for the alotted time.
Have you heard of virtual assistants? They are not as expensive as you might think, and may be able to help you with some little things that overwhelm you (even just 10 hours per month could help). some ideas: social media, bookkeeper, a person to respond to basic inquiries, etc.
Create a daily schedule and stick to it. Make it big and post it in your office and let your entire family know what it is.
Ask for help from family or friends. Are there other Moms who might be willing to trade days for driving your older kids to school/sports activities?
Take one day for yourself/family - meaning, make sure you schedule in something you enjoy whether it be reading in the bathtub or 1/2 hour or getting a manicure. the rest of the day should be spent enjoying your time as a family - playing.
Allow yourself one day to breathe deep, close your eyes, focus on your end goal. Then pull out a piece of paper and start writing down, in no particular order everything that you need to do each day, everything that needs to get done monthly/weekly, etc. Then number each item in order of importance. Take the top 10 items and start with the easiest one. Make sure to go back and cross it off when it's done. The sense of accomplishment will motivate you to move to the next item. After you've written your tasks down and organized them, you can then put them in your calendar, but keep that big piece of paper out and visual for the time being.
Good luck and remember, you're not alone.
-S.