Try the following exercise and you will see some results sooner than you think:
-make a nice warm-up
-Practice normally until you start to be tired. (is important that you practice normal stuff and not play nothing different than usual).
-Now Imagine for example that you start to be tired after 20 minutes, means you've found your breaking point...
-So, next day make a nice warm-up and just practice 15 minutes, rest 10 minutes, play 10 minutes, rest 10 minutes, play 5. This might seams weird but in fact you probably still not feeling tired and you've played 30 minutes (more 10 minutes than last day)
[Repeat this procedure for a week]
-Next week make a nice warm-up and just practice 20 minutes, rest 10 minutes, play 15 minutes, rest 10 minutes, play 10. Now you probably still not feeling tired and played 45 minutes (more 15 minutes than last week)
[Repeat this procedure for a week]
-On Next week make a nice warm-up and just practice 25 minutes, rest 10 minutes, play 20 minutes, rest 10 minutes play 15. Now you probably still not feeling tired and played 60 minutes (more 15 minutes than last week)
[Repeat this procedure for a week]
Continue on the same pattern on the following week until you get the desired amount of endurance.
When I say to rest 10 minutes doesn't necessary means to stop practicing. You can choose to take a break and have some orange juice, sing some passages, read a book etc...
If, for being sick or for any other reason, you have to stop playing for a few days, you should re-start this process again and find your breaking point.
Most important of all is, at least on your practice sessions, don't get to the point were you can't play more because you are already too tired. After finding your limits, practice until just before you reach them, and keep pushing your limits forward every week!
This is very simple, but it works for real!
PLEASE NOTE: Sometimes on a Concert, you have to play beyond your limits. It's a hard job, but some one has to do it...
But, at least on your daily practice, you should be able to control it.