Oh Mips and Three so sorry you are going through such a traumatic time. I know some posters are encouraging you to feel angry with your DP and pointing out all his short comings which are real enough. However as I have probably already said, all of your emotions are going to be mixed up and you are still very raw. You are experiencing the classic signs of bereavement following the loss you have suffered. Unfortunately all these emotions don't come in linear form, they dodge about, so one minute you are angry and then you are sad, and wish he would return, and then you feel lonely and so it goes on, the "tape" goes around and around in your head. Many people experience a "red hot" memory - so called because the memory is so painful it is like a burn, and then sadness and tears take over. Also grieving come in waves and sometimes you feel ok(ish) and then are overtaken by all sort of other emotions.
I honestly don't think people can tell you how to feel (and I'm not thinking of anyone in particular on the thread!) but I've found many MNs have very strong opinions on something and can be very definitive about what happened, the reasons it happened and what the future will hold!!
All you can do is take one day at a time and try to structure your time which will help a little. I still don't think blaming is the way forward, for either of you. I think human relationships are very complex things and dynamics between the couple tend to get set in concrete and we can't see for ourselves what is happening. It takes someone right out of the situation to help you understand some of the emotions you are experiencing. I think someone mentioned counselling for you, and whilst I think this is a good idea, I think you need more time to elapse to get the best out of counselling, as it isn't cheap - approx. £50 per hour dependent on where you live.
I think it's important for the two of you to keep the lines of communication open, try not to blame or re-hash what happened, as this will lead to friction and you don't need that now. Also I think it's really important that the children see you together and getting along with things in the usual way as this will help them to feel a little more secure.You need time to "lick your wounds" and so does he although he should most definitely have told you about his intentions, but maybe that was the only way he could do it, when you were away.
I still think depression is a torment and unless you have suffered from it, you can't have any understanding of how awful it can make you feel and there is still a stigma attached to mental illness.