Hi,
i'm a natural therapist and I may be able to shed some light on your problem.
The fact is that when you strain, you hold your breath, when you hold your breath, oxygen is not finding its way to your body's cells as is needed.
Concentrate on your breathing whenever you try to exert yourself. Many people don't even realise that they hold their breath as they are too busy doing something else to notice that they are not breathing.
Try that and see how you go.