I personally know I am much harsher on myself than Master ever has been. He needs only say I have disappointed Him, and the emotional ringer I can put myself through is worse than anything He could do.

That said, it helps to have Him assign punishments. When I am done with my punishment (usually writing a long essay on how I messed up, or being put in the corner for extended periods of time), I kneel before Him and know that He still loves me, and forgives me. If I picked my own punishments, I wouldn't have the comfort of knowing I had done what He felt was needed. I've had times where I've fidgeted too much or written a poor essay, and had to start over. Yet when He is satisfied, I know I have made a step towards redemption for my transgression. That brings me comfort, and is why I could never pick my own punishments.