Oh YesIAm - now you've made me cry! My kids are with their dad till Sunday and I am missing them like crazy. I spoke to them on the phone this morning and DS said 'how many more sleeps till I come back to you Mummy?' 
I did some research on shared care when I first separated from ex, and I read in several places that absences of more than 3 nights from the 'primary carer' is not good for young children - and by this they meant under the age of 3. Their attachment can be affected. As both you and your ex do significant portions of the care, I would say it isn't ideal for your son to be away from either of you for more than 3 nights. But you will have to see how it goes, I guess. And being very in tune to his emotional development and reactions will be key here.
DD used to cling to me screaming when ex turned up to collect her. As soon as I mentioned Daddy she would say 'no. DD no daddy'. DS was also badly affected (and he was 4).
I pray that you don't find that your DS has any reactions to this situation. If your child can accept it, then at least - even if you feel depressed and useless - you can console yourself that they are happy.
And I totally understand the feeling of dread as the time to handover approaches. I have the DCs from Fri morning through Weds morning on one of the two week cycles. By the time it gets to Sun evening, I am already feeling down because they are going back to their father on the Weds morning. It ends up spoiling the time I have with them - as much as I try to stop it from doing so.
People have said ' oh you will move on. You will appreciate the time you have without them'. But I don't. I don't want to go out. I don't want to meet new people. I want my kids.
I too thought when we separated that 'at least I have my job'. And it's true - I am financially independent and have bought my own house and support myself good job as my ex refuses to pay for anything despite earning 3 times what i do. But another job could have been got, I'm sure. Not as good as the one I have now. But to be honest, if I could have foreseen what has happened, I would have given up my job. Ex used my job against me in our court hearing - the fact that I worked (part-time) and therefore had to use child care negated the fact that I had always worked part-time and had had two periods of maternity leave with the children.