Sounds like you are out of shape and can't grind a point. You need to run and lose weight, but I have a feeling you won't committ to that.
One thing that will help instantly (besides having better footwork) is breathing right. I always inhale on opponent's contact and exhale on mine. It is just easy to remember and it keeps me focused.
You have to get your footwork right though for this to ever really pay off. It takes a few weeks of being tired and then suddenly you get it and you will find that you actually save tons of energy out there now that you are hitting with balanced form.
Please help TT Member BruceD get a wheelchair so he can play tennis again - http://tt.tennis-warehouse.com/showthread.php?t=496820