Exactly how desperate are you?
Suicidal?
Thinking of doing a runner?
Just want to curl up and for the whole damn lot of them to go away?
Wishing you had a different life and then feeling guilty because of DCs?
Please, please do admit to someone in RL, even The Samaritans, if you are feeling desperate - they are always there for you..or perhaps google mental health support telephone lines local to you.
Would it be possible for you to talk to DP about how you feel? Or even write him a letter (even though that might feel a bit wierd, it'sa fsntastic way of getting your thoughts down and not losing your train of thought, unlike a conversation)
I think a visit to a sympathetic GP would help too, it might be that some counselling would help you get things into more of a healthy perspective for you. Also have a look at the 'Living Life to the Full' website, which is by a NHS psychiatrist, and a really helpful place to start to think through some of your feelings about yourself, and how you cope, at this stressful time in your life.
Do you have a faith? Is there a ritual or prayer, or special piece of music that you can participate in or use just for ten minutes now, quietly, to calm yourself down and help?
I'm sure more posters will be along soon to reassure you that how you are feeling is really incredibly common in Mums and will have some good advice too.
You really are more precious than you are able to see at the moment x