This is an example of the "no fault 'se'" (or "accidental 'se'") construction, used when talking about something that is not your fault, like when you drop a glass (se me cayó el vaso = I dropped the glass (by accident)).
It may seem strange to us English speakers, who always say "I dropped" or "I forgot," but to the Spanish speaker, forgetting or dropping something is never their fault. The blame is placed elsewhere (the pronoun 'se'). The 'me' part of the construction expresses whom (indirect object) was affected by the falling or forgetting.
The literal translations I see bandied about for the English-wired brain (like "It forgot itself on me" or "It dropped itself on me") don't make sense. So, my advice is to just learn how to use the "no fault" construction and never take blame for accidents or forgetting something when using Spanish.
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