You need to go ahead wiith the divorce. Without a divorce, the legal system does not really get involved in a couple's domestic arrangements, except to prevent violence, abuse, etc. If you are the owner of the home (and your husband is not), you may be able to force the girlfriend and child out (evict them); if you and your husband jointly own the home, you may be able to force the sale of the home and distribution of the proceeds (if any), then find new living arragements; but that's about all you can do other than in the context of a divorce. A divorce, however, will legally and help to practically separate  you and your husband (and his girlfriend); distribute assets; provide for alimony or support, if appropriate; and generally get you out of a bad situation. You should retain a divorce attorney and move forward with it.