I remember when my daughter was younger that my husband could do no right. He was trying at his maturity level. We actually separated for 1 year and 7 months. I figured if I was on my own I may as well be on my own. Very angry by that point, btw.
It sounds like you are feeling the space/isolation that comes with the baby. You may have a moms group for friends, but they aren't giving you the closeness you want with your husband. If that sounds familiar, then here is what it took for my marriage to be reconciled.
1) Pass the 1st 6 months of infant time. I don't think I was able to "hear" much by then. We'd been married 5 years and I was so alone... with a baby now...and tired even though baby slept 5 hours at night. I wondered if he was cheating since he wasn't being affectionate in or out of the bedroom... nothing suspicious was going on in retrospect.
2) I read Dr Laura's book even though I'd given up during the separation...helped a little, but we were separated and it seemed like he didn't want me back. I'd recommend a different book first... see below...
3) Got counseling after my husband started being helpful and thoughtful... in case it was a "trick." He had tried to guilt me into moving back in with him to save money, because God hates divorce, it's best for the children...I felt like he was trying to manipulate me since he didn't want me back. 14 months later I started counseling for myself. Eventually husband was brought in. Our counselor said he was secretly thinking we were headed for divorce. My husband likes to be "right" so I very waspishly told him that he liked being right more than he loved me so we were done.
During a pause in counseling I found a book called Distant Partner by Dr. Les Carter. Based on what little you said and where you are at with the young baby, I would recommend you start with this book. When I read it, I kept thinking this guy had a camera in my home or something. I could so see "us."
http://www.marriagemissions.com/the-emotionally-distant-h... is an excerpt.
I learned I had reaction patterns. No matter what he said, I would react unhappy and blow up. He didn't stand a chance with me.
Eventually he became more involved and affectionate about the same time I learned how to keep myself from hyperreacting. My husband had been protecting himself and not allowing himself to love me because he was afraid I was going to leave him anyway... Insecure guy and affection (not sex) starved, aggressive woman do not make a good combination.
Long story short, I learned from the book Distant Partner. Saw some changes in my husband...Extended myself a little and saw my husband meet my changes...etc. until we got back together. I really hope you can find it in yourself to work things out. Yes, it's better for your son, but it's would also be better for you. I now have the husband I thought I married 10 years ago. My kids have their dad and he is so much more involved with baby #2 (who I call our "love child" when it's just us). Now I gladly do what's in Dr Laura's Proper Care and Feeding of Husbands.
Give yourself some slack. A large part of this may be hormonal, but you mentioned some insecurity so we may have enough similarities for this to help you. (My dad was unfaithful and so were both of my grandfathers...)
Please feel free to call me/email me. I was the exact same age as you when this happened in my marriage.