You will never truly forget anything. You subconscious mind stores all the data you experience. Now, you can take it out of your conscious mind by focusing on something else. As that focus and new thought occupies your conscious mind, less and less attention is given to the thought you do not want to recall. Eventually it slips out of your awareness.
Do not tell yourself not to think about something. The subconscious mind does not process negatives. So, when you think "Don't think this thought", the subconscious hears "Think about this thought." Therefore, you think about it. If the thought you don't want pops into your mind, just turn your awareness to something else. It will take time depending on how much mental energy you have put into the thought you don't want to have, but you can make it through.