I get really worn out when playing the trombone, how can I stop this?

It's just that playing a really long piece is hard because of the amount of air you have to push through the thing, and holding it up on my shoulder gets tiring after a while. My trombone teacher is getting me a tenor trombone instead of the b trombone I currently play, will this help at all? I was just wondering it it's normal to get exhausted from playing it?