Boxing stamina is built upover time. I truly feel that so many peolple who try to do too much damage themselves. Look if you are fighting a 4 or a 6 round fight train to fight 4 or 6 rounds. The pacing and the MANNER in which you exert your energy is very different than when you are fighting 10, 12, 15 rounds. The point is to be in peak condition to fight the number of rounds designated, not some other abstract of your own invention. You are better off spending the extra time learning boxing as opposed to running or lifting weight etc...
As to your dilemma specifically...relax. I'll bet that you're as tight as a drum when you work and that tension will exhaust you.