Mistakes are unavoidable and part of Drawabox is learning to work through them and create convincing images. In my view that means that the box still has to be a box, with lines meeting at corners. Don't write it off as a failure, think about which compromises will result in the most convincing box.

Here's a recent post showing that even with a ruler it's not possible to get the lines 100% accurate, but they're still very convincing boxes: https://drawabox.com/community/submission/3STUM6V . Don't think you're expected to create perfection when drawing freehand. Focus on being as accurate as possible when drawing your lines then do your best to work with whatever is on the page when planning subsequent lines. The better you do at the first part the easier the second will be.