Hi Mips - I think you are getting some very good advice/support now and no one seems to be blaming your DP any more which is a good thing and you must try to stop blaming yourself. As I've said I think your DP may think that having space may improve his depression, and stop him feeling guilty about inflicting misery upon you and the children.
You mentioned that he had acknowledged the letter you wrote and I don't know how your DP will react (but I'm sure you will know) some people would immediately write back and others will take time to reply ad not be sure of what to say, and yet others may not reply at all. In any event I don't think someone with depression is going to be together enough to reply to your letter in the near future. I imagine that you poured out all of your emotions in the letter and that might be difficult for him to read and assimilate.
VyB is giving you some very important "messages" to stop being so hard on yourself and I can only endorse what she says. You are suffering a major life crisis and it would be very strange if you weren't having some major reaction, it's called being human! I also think it was good that you went out and bought things for the kid's den. What ages are your children if you don't mind my asking.
I hope you can be comfortable with each other tomorrow though of course it is going to feel a little strange and even awkward to be "playing happy families" but I'm sure the birthday girl/boy will be pleased that you are both there for his/her special day.
Glad to hear that you have some RL support and with a best friend too - invaluable at times like this.
I read upthread that you had lost the thread. Just click on "I'm on" at the top of the page and it will bring up any threads that you are on!
I know I keep mentioning this but do you know why your DP won't see a GP about his depression - and do you know what the root cause of it is. Suppose it's out of the question that he would see a therapist - no I thought not!!