Why is Mary Warren scared to tell the truth about Abigail for herself and John in The Crucible?
By this time in the story, Mary is committed to the lies she has already participated in. If she admits to the court she has lied all of this time, and people have hanged for it, then she will surely be sent to jail, and perhaps killed herself.
To make matters worse, Mary is not a strong person and she fears Abigail, who has told her flat out of a "pointy reckoning" if any of the girls should recant their stories. Now she is in a no-win situation where she risks certain punishment on the one hand, or certain revenge on the other, and as a frightened young girl, she doesn't know what to do except continue in the lie, even after she has tried to come clean.