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Sounds like your not eating enough carbs and that is the "tiredness" you are experiencing. Do you think this could possibly be the case?
Also, you should always eat before cardio in my opinion or else you will have nothing to burn hence the "tiredness" which is actually counterproductive.
It is recommended to eat Carbs before exercise and immediately afterwards and then protein (shake) a bit later to rebuild.
If you want to try something that might help you get a bit of extra edge you can try the Vega Sport drink (not the VEga sport protein shake) immediately before exercise and during. Many people swear by this. Give it a shot.