I am sorry for worrying everyone - I posted yesterday because I have had similar situations in the past and got through them with help - this time is the first at home so I needed a reason not to move away from the computer until it was time to pick up the children.
I have been in touch with the Samaritans (by email - I could not be certain of being undisturbed for a phone call) and they unerringly pinpointed the major issues which I was too close to see - I am not sure I would have gone that far without everyone's help.
I did other things people suggested too; I played with my children until well after their 'normal' bedtime and then ds slept in my bed which all helped me over the immediate crisis.
I know it will not be easy to get 'me' back and posting on here is something that helps a lot - both for me and also hopefully in posting on other threads where through my experiences I may be able to help others.