No matter what, put your child first. Also, take some "me" time, go out with your friends, or family or whomever you can vent to, and take a bit of time to be sad or be angry or whatever your feelings are, but don't do it in front of your baby. And also don't say bad things about him in front of your baby.
Mine left when my child was that age and it was a relief for me. So much stress was gone. I was able to figure things out and how to manage without him. Don't dwell on the emotional part of "he's gone". Think about the good (that you can focus on a better life, get your life together, etc) Once you get things together he may want to come back. Just focus on you and the child.
If you're talking about just going out with the boys, that can be tough to handle, especially if you're at home all the time taking care of the baby. Just make sure you get time out too.
If you mean for good, let him go as hard as it sounds. If he's a man at all he'll realize everything he's got to lose and come back. Mine did that when our first was not even a month old. We had a fight and he said he was leaving. I begged him not to and he told me to let him go. The fight went out of me and I watched him leave then locked the door. My baby was screaming and I knew right then it was better DH was gone if he could be so mean. He came back later and tried to get in but the door was locked so he sat in his truck in the driveway until I saw him and let him in. It sucked and hurt like hell but sometimes you have to let them see it from the side of being without their family before they see reason.