I will make this short and sweet. Last year when September rolled around I had an advanced 1st grader, an advanced preschooler, a 2 year old who gave me a run for my money, and a 1 month old.
I KNOW EXACTLY HOW YOU FEEL!!!!!!!!!!! Well, although circumstances may be different here or there, I get the main fears and frustrations you are feeling at the moment.
Let it go! You will be able to start slow, just like I did, and your children will still come out of the school year happy and excelling. The baby will be your buddy, on your lap all the time! We had a bouncy seat that I just brought from room to room with me as I went about my day. I am not going to say it was easy, it was as far from easy as you can get. I will tell you that it was rewarding and it taught me to let go of some of my habits from the past. I tend to over-analyse and over-plan. A tiny baby made that impossible. Throw cloth diapering 2 babies and breastfeeding in and I had a disaster waiting to happen. After a lot of stress and frustration I just started to let go a little and things all fell into place.
If you feel God has put this on your heart, then please trust Him to make it work for you the way it should. He knows you best :)
This year I have a 'know-it-all' 2nd grader, a Kindergartener who feels like its a race to get to where his older brother is, a preschooler who is so hungry for learning, and newly walking 1-year old who is into EVERYTHING!!
I'm feeling some of the same stresses all over again, but I know the outcome already. We will have great days and we will have horrible days, and there will be lots of 'just made it by' days in between.
Have hope Mama, and believe in yourself! I believe in you!