You let me down.
let sb.down 让某人失望
至于LZ那个disappointed和disappointing 区别如下:
当主语为人时 disappoint 用 disappointed.
当主语为物时 disappoint 用 disappointing.
当修释词为adv.(副词)时disappoint(至于为什么adv.修释v.(动词)原因就不解了 国际规定我也没法)