It is not easy. Marriage is alot of push and pull. My dad came from a broken home and always has said (as a child especially), the consequences of divorce never, never, end. However, I recognize there is a place for that too. I believe that you are in a position to have this relationship turn around. The best thing you can do is look inward first. There are alot of books that can start you on the right path. I personally love The Power of Positive Thinking by Norman Vincent Peale. Have you seen the Secret? Similar thinking. Start by focusing on the good in your relationship and start by feeling those things, then express what you are grateful for in your marriage to your husband. Find ways to deepen your relationship. It may sound cheesy, but plan romantic dinners for 2 at home, leave little notes for him where he will find them throughout the day. Do something meaningful for him and expect nothing in return. You do this over time and it will become very fulfilling and rewarding to you. My husband sometimes becomes over focused on his life and often I find it is when I am complaining the most. Your husbands need to belong is probably more of a need of needing to feel valued. Find ways to show him how much you appreciate and benefit from the sacrifices he does make (even if they are few). Recognize the stress he has on his plate and ask what you can do to support.
As for his parents...any chance of inviting them over instead of you both going over there all the time. Invite them into your schedule so it isn't taking away from your life. Invite them for dinner or a Sunday afternoon when you take your baby to the park. If they are at your place, maybe they would understand if you slipped in for a nap while they played with baby or visited with hubby. You probably would have alot to gain if you developed a relationship with them. Ask them for advice. How you could better understand or help your husband. When you come from this perspective they will probably go out of their way to support your marriage in ways you may not recognize.
I am in no way trying to belittle your situation. I understand how challenging this can be. I also can see the greatness of it because you are on the verge of great growth and progression. Your soul can only stretch when you are pushing through hard times.
Lift up your chin and find gratitude that your husbands selfishness isn't taking him to worse situations. You have much to celebrate. Let go of your anger. Find compassion. Find love. Once you learn how to give it, you will see you will recieve more than enough in return.
I wish you much joy and peace in your life.
I hope this helps.