Secondly, you CAN cope without your husband. You are panicking right now but you will find a way to manage. Under no circumstances allow an abusive ex back into your life. You will be happier in the long run without him.
I don't have any kids myself, but I'm sure other board members will give you practical tips on coping with parenting and HMS.
I do have a few suggestions though:
Visit your GP, tell him/her that you are struggling, ask for some counselling and a referral to OT. OTs can help in numerous ways and they can give you some aids to help you practically cope around the house and with your baby/toddler. Also ask about pain management clinics, and whether there's any provision for free practical help on the NHS (such as a home help).
Apply for DLA - it's a stressful process but you might get quite a bit of money, which you can use to pay for a cleaner or childminder, for example. There should be a Welfare Rights service in your town which can advise you and help with applications.
School run - would it be worth getting in touch with your children's friends' parents and trying to organise a car pool, or just asking them to give your child a lift in the mornings? Even one day a week without the school run would help.
I understand how you don't want to lean on your older kids... however just talk to them about your fears, I'm sure they will reassure you that they'd rather help you out than see your ex come back!
I hope this has helped even a teeny bit!