Sports are tiring for a reason. They weed out "the weak" and "the strong". In other words, they find who's a better fit for that sport. No offense to anyone, but that's the truth.
Even if you find that you aren't fit for that sport, you shouldn't quit. You know what they say: "Winners never quit, and quitters never win!"
Sports not only whip you into shape, but it also helps your brain somehow. I just know that studies show that the grades of those who do sports are better than those who don't. Or something of that sort.
Also, if you are tired after every practice, and I mean tired as in you can't do anything tired, then that means that something isn't right. Maybe you aren't working hard enough on most practices, and when you do try, your muscles aren't used to the strain and you just get very tired. Maybe you were built for a different sport.
All you have to do is try.
Always try your hardest in sports - you never know what might happen. There have been people in track who started off as very slow people, but they worked hard, and because the fastest person in track. There have been people in track who started as the fastest and didn't work on anything or try, and in the end, became the slowest person in track.
If you just give up, you might never become that fast person. Sports cause as much mental strain as physical strain, because with most sports, you have to encourage yourself to "keep going," like in cross country.
In cross country running, you have to literally FORCE yourself to run. Everything hurts: your legs, your muscles, your throat, etc. But just keep thinking that you HAVE TO go on.
As Jason Mraz says:
Well I won't give up...
Don't give up. Keep going. Stay strong.