Disillusioned is a word that has a very specific meaning in English and isn't a direct synonym to disappointed. The idea has a sense of losing hope. Someone who is "disillusioned" was once "idealistic" but had their beliefs crushed, destroyed, or otherwise lost. It is associated with "apathy".

The word "disillusioned" is built from the word "illusion." The idea is that one held a previous idea or set of values that were unreal (illusion), but now the person has had these unreal ideas removed (dis-illusioned). It is often an unpleasant experience, but many writers will use it in a context where the removal of these false ideas or values is a good thing in the long term.