Okay so that is a really tricky question that most Spanish students couldn't even answer. It is not something we think about when we talk and it is a matter of ''syntax''. In that case the verb is ''olvidarse'', which is not the same as ''olvidar''(there is a really really small difference in terms of meaning). By the way, as far as I know this is only for Spanish from Spain. I think the usage of this verbs may vary a little in some regions of South America. "olvidar" is transitive, so it needs a direct object: "olvidé las llaves" "olvidarse" is intransitive, and it needs the preposition "de" (Complemento de Régimen in Spanish). It is a ''verbo pronominal'', which means it always goes with a pronouns. Your case is a bit different because the use another trickier form with two pronouns. Here, the subject is actually the object you forget, and the indirect object is the person who forgets. This may be a little trickier, but it might help you. When you'd like to say ''a mi mismo'' or ''mutuamente'' you may change it for just ''se''. I hope I made it a bit more clear for you. I am sorry if I did not, it is a hard question and it's something we learn since we start speaking Spanish, so don't even realize what's going on but know how to use it. Good luck!

Me esfuerzo al máximo para dominar el español. Lo estoy haciendo lo mejor que puedo para dominar el español. Lo hago lo mejor que puedo para dominar el español. Lo intento al máximo para dominar el español... Lo intento todo lo que puedo para dominar el español...

You may consider coming to Spain as an au-pair!