Michael Henchard's heroic tragic flaw is related to Hardy's theme of how a person's inner character traits interact with blind Fate. Hardy's point is that for every choice a person makes, future choices are either broadened or narrowed and Fate is either advanced or forestalled. An example of this is that early on, Henchard resisted disencumbering himself of Susan and the baby, even though he believed they were holding him back. With these moral choices, Fate was forestalled. Had Henchard continued in this path of moral choice, Hardy suggests, his end in life would have been a different one. Conversely, at the drunken moment that he agrees to a drunken opportunity to sell Susan and their baby, Henchard makes a decision that so curses and narrows his future choices that a blind destructive Fate is aided and hastened. It takes some mental effort to keep the threads of Fate and Choice separate, especially when Hardy is showing their interaction, but the message is that had Henchard displayed different character traits, Fate would have brought forth different opportunities, or the opportunities Fate did bring forth would have been rendered powerless.
That being said, Henchard's tragic character flaw is the dichotomy within his nature. He is honest to a fault as seen when he doesn't deny the furmity woman's accusations. Yet he is also self-protective to a fault as seen when he disencumbers himself of his family because they are holding him back and when he lies to Newson by saying Elizabeth-Jane is dead. He is faithful to his word and always conducts his business on the highest standards. Yet he can explosively declare rash and cruel things the pursue them as though they were reasonable and right things, again as in the sale of Susan and the baby. He has determination and will power and can keep a promise for years and decades. Yet he can't think with foresight and foresee disastrous ramifications of impulsive thoughtless acts. He has a deep sense of justice, as in his attitude toward himself when his is alone and forsaken at the end of his life. Yet he is petty and mean spirited as was the case in his feelings toward the loving and accepting Elizabeth-Jane the morning after the two paternity revelations. It is this unintegrated duality, this combination of light and dark, that constitutes Henchard's tragic flaw: though he has high values, he lacks the ability to think through and foresee future affects of present choices and acts.
A good thesis statement might incorporate this dichotomy and duality, this combination of light and dark impulses, and relate it to Hardy's theme of interaction between personal choice and Fate. Possibly something like this might work: Henchard's tragic flaw is the dark versus light dichotomy of his nature that proves character-determined personal choice spurs on or forstalls the realization of Fate.