If we run away from our pain in life, we are running away from something that has great potential. In zazen pain often manifests in the emotions and in the body that is responding to those emotions, especially when those emtions are not in full awareness. The body may be acting to mask the emotions that we don't want to look at.
What was prominent in today's zazen is that it is through pain and struggle that the "best" spiritual growth comes. Therefore, I need to accept and witness and use any discomfort or pain as a tool for concentration and focus, allowing my awareness to expand.
Awareness needs first of all to move through my karma. Karma is stuff that I have not let go of.. It is easy to let go of pleasurable situations, yet the struggle to build the "spiritual muscle" come more effectively from the pain encountered in zazen. If I do not resist my pain, if I accept it, and go deeply into the core of it and from there, I can release it, and expand my awareness a little more. Undoing karma? Possibly, but there needs to be no goal or achievement in Zen.