Rambam - Sefer HaMitzvos: Negative Mitzvah 356: Negative Mitzvah 356

Negative Mitzvah 356: It is forbidden to remarry one's divorced wife who married and divorced a second time
Deuteronomy 24:4 "Then her former (first) husband who sent her away may not take her again to be his wife"

A man who divorces his wife is forbidden to remarry her if she married another man and then, was divorced a second time or was widowed from her second husband.

(If she did not marry another man, he is permitted to marry her again.)

When things don't work out, trust in G-d and stay calm. Even if it's all your fault and you deserve everything you're getting, trust in G-d that it is all for the good, and stay calm. When He sees how much you trust in Him, He will make it for the good.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com