It's alright, really, let yourself and feel it as much as you can bear...it will come and go in waves, phases, days...it's being processed and every bit of feeling or thinking brings you a bit further to being alright again.
I know it must hurt like hell, I thought I would actually die when DS1's daddy left us and I found he was with someone else.
I couldn't function, just walked round in a daze because nothing else mattered to me...and we were only together 4 years.
I second what someone said about it being a good thing in a way that you are so sad about it...I left my second child's father in November and all I can think is I wish he would find somebody else, I hate him and am not sure I ever loved him...just hoped and thought we had something worthwhile.
Now I feel nothing for him, and that's really sad as we have had a child.
For all the pain, I know I loved DS1's dad, and that was worth having even if I lost him eventually.
HTH, be gentle with yourself.