Joe on training.I'm very old fashioned in my views but as soon as I see a fighter, trained by modern methods and with the modern cluster of "specialists" in his camp, that can fight 15 like they used to I'll change my views.
Diet...Lord knows I'm clueless here but I know this. The best conditioned, strongest fighters I've ever seen were poor and poor people eat rice and beans. My experience mostly refers to fighters from Mexico- that is who I grew up watching and reading about- and rice and beans is a staple of the diet there. Emanuel Steward says the same thing; he says he cooks his fighters collard greens, beans and chicken. Jackie McCoy, the great trainer, called beans and rice "earth food" and swore that they made for strong fighters.
Breathing is a curious subject...Lots of fighters forget to breath, especially when they get excited, like in the middle of an exchange or when things are going very well or very badly. This is often an ignored subject but here is what I think, and I learned this from a 69 year old Australian opera singer. She was very scrappy despite being 4' 11" and weighing maybe 98 pounds. (google Elizabeth Sabine).
You breathe through your lungs. Breathe in short gulps of air; the most common way of running out of breath is not from lacking air but from having too much. If you breathe deeply but don't use up and exhale that breath, then your lungs get clogged with "bad air" and you can no longer take full breaths. So take short breaths and exhale fully- like when you punch for instance and don't forget to breathe. Doing this, you can keep your abdomen tight- so body punches don't knock the wind out of you- while moving and breathing easily.
The last reason I think you, like many others on this site, may lack stamina, is that you train to much. Look, the early morning 4 miles of roadwork is as much to build discipline as any thing else. Rocky Marciano didn't run millions of miles but he walked every place he went. Running- alternating sprints, jogging and brisk walking- is to sharpen your wind. You should train every day and not just to prepare for a fight. Stretch in the morning every morning. Run-as explained above- three times a week, or on days you don't spar. Don't jump rope on days when you run or spar- it is over using the legs. When training to fight 4 rounds, train to fight 4, not 10 or your body will not peak properly.
See, training for a fight is not something you begin to do when you learn about a fight. It is something you begin to do the day you begin boxing. A good, hard, sharp hour to 90 minutes a day is fine. Working 6-8 hours a day is foolish. And just get me started on cross training!