You need to tailor your workout to what you can realistically do and to the shape you're in. If you try to do too much, you will get tired, and the training won't have much effect.
A lot of people forget that recovery is as important as the actual training; if you're getting tired often, you may be doing too much, or doing too much else, which has the same effect.
When I was with the ski team, we scheduled hard workouts for the morning, so the athletes would be fresh enough to perform/train well.
That said, with experience you will learn whether on a given day you can power through a session when you're tired out, and when you cant. An elevated heart-rate will indicate that you really are too tired to do much - and then you do quit, or don't start in the first place.