The moral lesson is forgiveness and, as the title says, remembering. The speaker asks to be remembered after her death, when physical contact is no longer possible; when decisions can no longer be made then undone; she reminds the beloved that there will be no more future to plan, no more dreams to dream. After the physical death, there is no purpose in counseling or prayer to make things better between them.
The speaker also knows that in time, the beloved will probably occasionally forget about her, but that it is normal to do so; when he recalls that she has for a time slipped from his memory, she urges him to not feel guilty about the lapse.
Finally, after death, their problems, small and large, no longer exist or have any relevancy. Since this is the case, the speaker wants the lover to forget those troubled times and smile. When he does once again remember her after those forgetful periods, she hopes that instead of the dark times, he will only bring to mind the happy ones.