I made an appointment, but they made me wait for a long time!
「予約したのにすごい待たされた」は次のように言うことができます。
ーI made an appointment, but they made me wait for a long time!
例：
I made an appointment at the doctor's office, but they made me wait for an hour! What's the point of having an appointment?
「病院で予約したのに1時間も待たされた！予約の意味がないよね？」
to make an appointment「予約する」
to make someone wait「人を待たせる」
ご参考まで！
カナダ 
