What you are feeling is normal, I remember when my husband and I seperated years ago, it was right before the holidays, and I like you really just wanted to be left alone and pretend like they didn't exist, but....I had babies and they deserved everything the holidays represent, no differently then my single mom did for us girls growing up, so I put on a happy face during the day, and at night in my quite time I had my pity moments.
This will pass w/ time, but right know all you have is this moment w/ your precious little boy, who will not be 3 1/2 again, pull up your socks and redirect your feelings into something with more meaning.....him and his childhood memories, the only childhood he has, go out buy youself a silly little santa hat, or some reindeer antlers, and let that be your reminder every time you start to feel the blues, that there is a bigger picture then just you, your son.
You can right to me and vent if you would like, I wish you and your son the Best of holidays!