How to deal with a complete failure in a group research? I’ve led a small research project with another researcher. I’ve proposed a certain approach to a problem, but in fact, this approach doesn’t work, as even its simplest toy version doesn’t work. In retrospect, this toy problem should have been tested right after my part of algorithm was completed, so that two months of my co-worker’s subsequent effort wouldn’t be wasted. But coming up with such a toy problem wouldn’t be possible given the level of our experience, and especially this is my first group project.

Is this kind of situation common? How do I deal with it?
Failure is intrinsic to science: how can you think of exploring the unknown without ever failing? Alas, it's a certain modern, distorted, entrepeneurship view of science that brought us thinking that science should always be successful.

For my master's thesis, my PhD thesis and part of my subsequent research, I worked on an experiment that eventually didn't work. Ten years. But I learned a lot, and now that more than ten years have passed from the moment I realized the failure, I don't regret it.

So, don't be discouraged and focus on what you have learnt and not on the failure.