Every one of us has memories of past whether good or bad. How those memories affect us and should be treated has been abundantly explained by scholars in various ways under the famous Law of Attraction. One advice says "remember good things of the past and be proud of those", while another says "forget about the past and move ahead". Both are fine and interwoven.
Apparently all the three periods of life sound different but are actually interrelated and cannot be separated. The fact is that we shape our thoughts and our thoughts shape our lives. What we sowed in the past, we are reaping it now and what we sow now will be reaped in the future. That is how our past shapes the present and present build the future. It is great to forget the past but when a person consciously tries to do so, practically he cannot forget all of it. While trying to remember only good things of the past, one naturally recalls bad things as well because memories are not something physical that can be sieved out to retain only the desirable material. Thus some of the past carries itself over to the future through the present.
It is not possible to completely forget the past but what is possible is that one should not dwell on bad memories of the past. Do not cling to them because doing so will not help in any way – you would only force yourself in to depression to whatever degree and length of time. Whenever such thoughts come to the mind, treat them lightly and refrain from going in to details of what had happened in the past because if a thought is forced out of the mind, it returns with greater strength. Try to switch to good things. This is what the first advice referred to in first paragraph means.
All of us plan for the future for both minor events like eating something particular or major events like taking up a new job or starting a new business etc. What you think and plan are the seeds that will grow your future tree of life. Plan it carefully, do not repeat the mistakes of the past and do not be over ambitious. This is what the second advice referred to in the first paragraph means. Learn from the past and move ahead with new courage and strength.