Are some people naturally fitter than others?

[RightCross94]

Maybe you aren't relaxing enough, that's why you are tired. And often worrying about getting tired can make you tired, just don't stress and keep working your fitness.

[boxing_great]

thanks for the comments.
I been sparring since the late 90's on and off. So I have heaps of experience.
Before I jump in the ring, I do get nervous , butterflies sometimes and when I am in the ring.
So I think that might contribute to the tiredness.Also I might be loading too much with my punches without knowing it,I'm not sure, because I find when I throw a punch and miss with it, it does take A LOT outta me..
As for fitness work, I am starting to skip regularly, but I find working on your stamina is the most boring thing about boxing.I really hate it, and also it is really difficult because you always go through that tired stage and u feel SO PUFFED OUT, its very difficult to kind of deal with .