尊敬していた先輩の無責任な態度に幻滅した。
I was disillusioned by the irresponsible attitude of a senior colleague I had looked up to.
憧れの都会暮らしも、実際に始めてみると幻滅の連続だった。
The city life I had longed for turned out to be one disappointment after another once I actually started living it.
そんなことを言ったら、ファンを幻滅させてしまうよ。
If you say something like that, you'll disillusion your fans.