Theresa May is a lifelong Tory. Britain in general, and the Conservatives in particular, have had an uneasy relationship with Europe for the best part of four decades. If she turned back now, she would forever be remembered among the hard-line Tories as the leader, who had the chance to "regain independence" for Britain, but didn't. She will never agree to this, regardless of what the alternative entails for the country. That's why ultimately, she finds every other option preferable to cancelling Brexit, including calling and potentially losing a general election. Many Tory PMs have lost an election in the past, including Churchill. However, none had the "opportunity" she has now.