Thank you for your question. This is a difficult decision to make, and I have not had much involvement with co-ops, but here is my advice based upon your situation.
Your children are young and probably not feeling a tremendous need for frequent socialization at this point. They may want that in a few years. Right now, the time that you can give to them is valuable. Co-ops are valuable, too, but I'm guessing that you did not decide to home school in order to be available for other children but for your own.
You may be letting down other families, but your primary responsibility is your own family. It's easy to put every waking moment into home schooling, and I understand that many wonderful co-ops and other support groups would not be able to exist without the commitment and sacrifice of key members.
I advise you to focus on your children's learning. As a new home schooling mother, you need to feel confident regarding what your children are learning. The co-op may fall apart (and many do) or it may grow from year to year. As you become more confident and gain experience, then you may decide to form a new co-op or to join an existing one. Give yourself a chance to learn this new role as a home schooling parent.
God bless you and grant you peace regarding your decision in this matter.