Is this Frazzled Parent Syndrome or something requiring more thought?

I think you need some respite. Someone in your extended family perhaps or the father needs to give you some time to be alone. Even if it's simply an hour a day. A two and four year old together is enough to stagger a parent if there are no other extenuating circumstances. I think that the fact that the 4 year old is able to keep it together during preschool speaks volumes. Children who have been abandoned or have experienced issues that tap out a parent often exhibit behaviours that are symbiotic with the level of stress the parent is feeling. Out of necessity these children develop a sixth sense which served them well to tune into the people giving care. When you think about it; it's self preservation. I believe that most mothers and infants develop a sort of communication that exists in some form undetectable to other people. It happens with colic. I've talked to people who had to deal with colic and it seems that when the primary caregiver was away the colic was easier to manage. It's like there is a hum of stress the child picks up on so it's helpful if the mother can detach and look after her stress. Even if it's for a short period of time. Feeling overwhelmed is not a sign of weakness or poor parenting; it's normal. If you can time yourself out. Tell the kids you are taking five minutes to regroup. Call it what ever you want. Tell them you need to close you eyes whatever. See how that works. If you can get them out of familiar territory. A walk where their focus is not you. Anything. Try having a picnic in your living room rather than eating at the same place. Shift things around so that they aren't triggered by routine. It helps sometimes. I remember once making a tent in the living room and sitting in there; telling my daughter I was on a time out pretending I was at the beach when she was small and driving me nuts. It worked. She became fascinated with the concept of seeing me pretend. I talked to her afterward about how good it felt. She took a turn at it. Do you read to them? Can they sit still long enough to engage in a story. That can set the tone to help them focus outside the present.