It's a pronoun, it is refering to the person who was told the thing. The sentence in English is "It disappointed me so much that you told him", in this case SE is the translation for HIM. Without it the sentence would be "It disappointed me so much that you told it".

Hi Charles, The "se" makes the verb pronominal. In other words, it's specifying to which person is the action done (like a pronoun would). If you say: *1) Me ha decepcionado muchísimo que se lo hayas dicho (your disappointment comes because the person you're addressing said that to him/her) *2) Me ha decepcionado muchísimo que me lo hayas dicho (because that person said that to me) *3) Me ha decepcionado muchísimo que lo hayas dicho (simply and merely because that person said that, regardless of to whom) Please notice in case 1 you may be sad, for instance, because that person (the receiver of "se") would be offended if that is said to him/her, or because you can no longer give him/her a surprise (just examples), whereas in case 3 the single fact that the person expressed that words/thought/etc. is what makes you upset. HTH! :)

(se )means that you are refering which person is who receive the action. or you can say: -Me duele mucho que le hayas dicho(podria ser un secreto,etc a alguien) . -Me ha decepcionado por lo que le has dicho.