Hi lily, Everything in life have possitive and negative sides, the difference just what side is much than the other. Every person has different idear and way to think, on your question many of people could say the first love can't be forgotten. Maybe its true and maybe its not. But i want to answer that on my way the way i think and believe, the way life it is, like a stomach when you feel hungry you eat then after couple of hours you hungry again so you have to eat, you forget what you eat before even if it was nice.
The other idear is a person can not keep the memory forever even if its still there, what i want to say here is a memory of yesterday is not the same strong like a memory of last year and a memory of last year is not same like of that before 20 years.
We can think how death is hard for everybody especially when he or she lose someone near like a family or children, the saddness will not be the same forever, everyday it fade till it become something normal.
So by these idears i say we can not only forget our first love but we can forget everything. The matter is time, everything by time and through time changed, Even the impossibles.