Why do I get tired in less then a minute when sparring If train real hard?

It could be because when you spar you have to move and follow your opponent. When you do the punch bag you are not moving really you are hitting the bag but in reality the opponent is not going to be a punchbag standing still.
I would work on movement

 

Mental exhaustion. Look it up Cus D'amato and Mike Tyson talk about it

 

Yes, it's getting comfortable in the ring and being relaxed that will help the OP. I'd bet it's due to tension.:good

 

as others said its nerves you probably tense up

do you pace yourself on the bag? do you pace yourself in the ring?

How much do you move when doing bag work?

could also be that you are taking punches and blocking punches and not breathing properly while one defense

 

Same reason why a person can spar 4 - 3 minute rounds and be tired but not exhausted but when in an actual 3 - 1 minute round amateur bout are completely gassed half way through the second round. Nerves, adrenaline rush and loss, with the addition of muscle tension all add to energy and stamina drain. Learning to relax and pace ones self can lessen the drain.

We took one of our pro fighters to a show in New Mexico. He sparred 16 hard rounds a week before the show. He was tired but not completely exhausted. At the show he was so stressed about fighting at a first time career higher weight and opponent level he was exhausted and TKOed in the second round.

We get all sorts of trained athletes who come to our gym thinking they are way passed physically than anything we can throw at them. Most are puking before training is over.

Getting comfortable and relaxed in the ring will help. You just need to spar as often as possible.

 

Some newbies hold their breath when they throw punches. They know it's wrong, but they do it involuntarily when exerting force. Same way you lift a heavy object. You hold your breath for a second until it's comfortably lifted. All that exertion when trying to throw a punch gasses you out real fast.
And like everyone else said, tension. You just gotta relax in there and get into a comfortable rhythm.
TRruthfully, it's nothing but repetition and practice.

 

In sparring, your often throwing punches quicker to try and hit your opponent. New people tend to try and overcompensate for not being as fast so they try to hit harder, generally never works out for them.
Taking hits and missing is also more exhausting then just landing punches.

 

Try to relax with good footwork to start even drop your hands when out of opponents range, sounds like your too stiff and rigid, loosen up !! And things will flow !!

 

Nerves...or you don't go hard on the bags so you don't really have stamina. Spar more.