I don't understand when to use "mă" and when to use "îmi"
It just depends on whether the verb is dative ("to" object) or not. You can think of aminti as meaning "recall", so I recall to myself.
Din păcate nu este atât de ușor... (Întreabă-ţi prietenul de ce ...!)
Why not "I remind myself of you"?
It makes sense in literal translation, but colloquially that sounds pretty awkward and could be taken to mean, "when I think about myself, it reminds me of you".