What happened is that you rushed into a marriage with a man you did not know. You did not take the time to know who you were marrying. You knew him for only three months before he proposed and were married only six months later. This is not enough time to get to know a person before you decide to get married.
You also said this was a “whirlwind” relationship presumably meaning you were “head over heals” in love. This type of love is always dangerous because people tend to overlook elements of a person’s personality or behavior that would normally alarm them. You may have become “blinded” to problems in the relationship that might have otherwise concerned you. This may be what happened in your situation.
Since I only am getting one side of the story I do not know if the divorce was “your fault” as your ex-husband claims. Based on your letter, it certainly seems as if he was the one who ruined the marriage. He lied to you, possibly cheated on you, stole your money, and much more. If this was the case, then it would have been his behavior that destroyed the marriage.
But even if the divorce was “your fault,” and I said above, it certainly does not seem like it was, maybe it was the best possible outcome for you. Once you got to know the man you married, you learned that that he was a liar, and a thief who is now trying to gain custody of your daughter. This is not a man you want to be married to. Your decision to get a divorce seems more like a blessing than something you should feel guilty about.
What should concern you is why you married this man to begin with. Don’t feel guilty about deciding to divorce someone who betrayed you and much more. Rather, focus your concern and energy on trying to assess what led you to marry someone who you did not know and who turned out to be a liar and a thief. A therapist can help you greatly with assessing what led you into this type of relationship, and more importantly, to help you never make such a costly mistake again.