The Blancos skipper admits he has suffered after being consigned to the bench for five months by the Portuguese coach last season.
The Spain international was ostracized during the second half of last season, and forced to spend five months sitting on the bench after losing his place to Diego Lopez through injury.
The demotion of a club icon such as Casillas to the role of the reserve caused great consternation in the Spanish capital and the legendary goalkeeper freely admits that the situation affected him deeply.
"I've cried, I've suffered, I've been unwell. I've had sleepless nights," Casillas admitted in an interview with FIFA.com.
Casillas admitted that so upset was he by the situation that he considered leaving the club he has represented for the entirety of his professional career before Mourinho eventually departed for Chelsea.
"I'm a 'Madridista' right down to my soul and above all else, what matters to me is the club," he explained.
"I would love to retire at Real Madrid, but I will not make problems if a coach does not have me."
Casillas has retained his No.1 spot for Spain in the Confederations Cup this summer despite his lack of playing time for his club, and he confesses it has been a relief to be playing again.
"I have regained happiness," he enthused. "It was not easy but I'm another Iker Casillas now.
"After five months, it takes some time until everything comes back to you, but, luckily, I have had a great deal of help of my teammates and coach [Vicente del Bosque] and that's important when it comes to having to start over again."