You have probably gone through something traumatizing and want for it to leave your memory entirely .While this is a perfectly human reaction and you should not worry if you feel this way about something particularly harmful to you, you should definitely realize that this attitude is not a healthy one and it is not one that you should be okay with having for a very long time. Once the memory is over, it is over and you should not allow it to continue to fester and to harm you no matter how painful the memory is.
When you feel like you are stuck in this kind of situation and are constantly having this recurring dream, it usually means you need to talk to someone about what you are feeling, get some advice from a friend or (if you can) a professional. This is a great way to actually work through the reasoning that you have in your mind that makes you remember this painful experience. Talking to someone about your memories can be a great outlet for you when you are in a sticky situation.
Some generally good advice would be to try to own the memory. Recognize the fact that it was very influential to you and very important as well (even if this is in a negative way). Then realize that you would not be the person that you are today if everything that had happened to you had not happened to you in the exact way that it did. This includes that same memory that is giving you so much trouble. When you are having dreams that you have amnesia, this is a sign that you need to come to terms with your past before your mind goes into repression, pushing the memories back into your mind to interfere with your subconscious and the way that you operate at a more basic level. This would be even worse for you.
Alternatively if you dream that you have amnesia it can be a warning that you are not doing enough to hold onto your good memories. Many people take pictures to try and capture their memories. As much as many new age people try to profess to the idea of living in the moment, when it comes right down to it, forming memories is what the human mind is all about and it is a joy for us to do so. We love remembering the good times. Having a good recollection of the good times in our lives can be extremely helpful, especially during the worst times where we can go back and access these memory banks to try and make ourselves a little bit happier in the present by looking through the lens of the past. If you dream that you are having amnesia, take it as a warning that you need to make some changes.