Sorry to hear about the post AC/interview rejections guys. I know these can be especially painful as by that point you’ve invested a lot in the firm, met the people and probably got your hopes up.
I also know these motivational posts can lose their effect after a while, but I’d just point out that to even get an interview at these kinds of places puts you at the top X% of applicants, so you must be doing something right. Get feedback if you can (although it’s not always entirely helpful), write up the questions you struggled with and try to reflect on the interview. If you fell down on an M&A question, make sure you can ace that question the next time. If it was motivation, think about how you can really convince a firm that you want to work there.
All that said, remember, it’s not always the case that you did anything wrong and quite possible you were a strong candidate (maybe you just didn’t click with one particular partner).
Finally, let yourself grieve for a little while (I used to fight that but it’s completely ok to be upset!), give yourself a little break and then put that energy into the next one. I really think so much of it is about interview technique, so if you keep practicing those questions (which firms quite often repeat), you should be getting closer and closer to your goal.