Given the resets and extensions you used, this was likely too high of a start for you: You failed 300x5 and then dropped only 10lb to start the new round. Remember also, TM is not for everyone. In fact it is not for most people. You may likely need to adjust further to not run yourself into the ground (i.e., volume SQ 5x5 is too much for most people).
For your SQ, you need to lean over more at the start.