The opposition to her proposed deal with the EU comes from left, right and center, from a variety of motives.

I'm not a great fan of Corbyn (though I've defended him from what I felt were unwarranted attacks on the UK forum in the past). He's currently playing at being as much of a Brexiter as May is - in fact, his main challenge recently has been that her negotiated version of Brexit isn't hard enough to fulfil the "wishes" of the majority who voted for it - and what alternative plans he and the Labour Party front bench have made public so far seem little or no improvement on what May's offering, and probably have less chance of being accepted by the EU.

If May does stand down next week, it might trigger an election, but it's more likely it might only trigger a Tory leadership contest. The timescales required for either aren't favorable in terms of a better outcome from the Brexit negotiations.