Is there a way to make yourself have amnesia and forget something bad?

Perhaps: This is an area of active research. No standard methods exist, but investigators are working on ways to help people forget bad memories that may cause post traumatic stress syndrome. Several research studies have shown some success. For more on this topic, visit: http://www.Scientificamerican.Com/article.Cfm?Id=memory-brain-tour-video.

Several ways: Try cognitive-behavioral therapy or emdr for starters. Ericksonian hypnosis, with an expert therapist, like stephen gilligan, is worth a try. Mindfulness based cognitive therapy or tm may pay benefits in all areas of one's life.

Rather, neutralize..: Rather than forget, the usual goal of the healing process is to neutralize the emotional effect of that bad experience/memory & become emotionally at ease when the memory/thought comes to mind. On this, plenty of research & writings are out there. Usually we try bringing things into the conscious realm rather than the other way around... So that we deliberately make peace with what happened.