Have you ever been into jogging? If so, then you'll recognize the feeling I describe - when you finish a run, your body is covered in sweat, you're exhausted, your muscles are aching and you feel like you can't move another inch...and it feels so very good.
You've pushed yourself to your limits and beyond, and that just leaves you feeling satisfied on a very deep level.
However, that feeling of satisfaction is just a pale shadow of the satisfaction our souls feel when we push ourselves in the spiritual realm. And although we live in the physical world, we still feel some of that shine...Remember that you're living life truly, the way you want to, the way hashem wants you to. Focus on how good that makes you feel. And remind yourself how much of an accomplishment that is, especially when you're feeling sick and weak.