Let's imagine someone has a girlfriend who's his one and only and means the world to him. She breaks up with the guy for whatever reason (say she sees no future in that relationship.) "While he has nothing to lose," he decide to propose her. Prior to this attempt, he says to himself: "What have I got to lose?" The answer would be: "nothing!" So she's either going to accept my proposal (which would be an added bonus) or reject it (which won't make any difference compared to the time being.) However, it can't get any worse so I'm gonna take my chances. There is a proverb in my mother tongue: "There is no color darker than black" which means: there is no any worse situation than the current one! The logic behind this saying is that when things can't get any worse, and someone has hit rock bottom, there is no worry to lose (because there is no more positive things to be lost.) Therefore, whereas there is no hope for any improvement in the conditions he should risk it all and like they say: roll the dice. He can go further and not be scared of any worse happening or any probable bad event lying ahead.
- It's always darkest before dawn. (But it is indicative of a positive situation while it sort of gives you hope that things are going to change and get better. The expression that I'm looking for has a negative meaning. It describes a quite bad situation that can't possibly get worse. So you have nothing to lose and can keep going down the way you're in.)
Please let me know if there is any English saying for this case especially in AE.