Dealing with the constipation will probably be key, but other things can be a stop gap while you work on that.
Like chewing parsley, or not letting your mouth get dry. Plenty of water can help breath and constipation. (I'm not one to talk about drinking lots of water though!) Also flossing and scrubbing your tongue with the toothbrush can clean your mouth of bacteria which may cause smells and tastes.
Maybe you could get checked for any underlying causes?