Hi there!
The difference between the trail and the treadmilll is simple but not always obvious! The treadmill keeps you at the same level - or a preset level - all the time. If you have done a lot of treadie and then hit the trail, you will feel tired quite quickly because even though the trail might look flat, it won't be - even on asphalt or concrete which is set according to the earth terrain and camber. Two ways you can overcome this is to alternate your treadie training with your trail (ie a day on the treadmill the next day on the trail) and a REALLY good way of transitioning to the trail is stretching. When you're on the treadmill your muscles, ligaments and hamstrings become set and are not stretching to cope with change.
If you are able to regularly and evenly alternate your treadie and trail that will alleviate your tiredness. If not - if you only trail at the weekends for example, then make sure you programme a good 15 minute stretch regime after your treadie training - particularly calf, hamstring and glute stretches.
Try it and see how you go! I have done a lot of cross country running but was limited to training on the treadmill during the week and until a very clued up trainer pointed this out to me I suffered the same thing. And it wasn't very good for my confidence in competition. Good luck and good training!