Yes, this was a result of the stress of his career. He did not know how to stop himself from living a risky life. His career gave him great highs and great lows and he had no other way of coping than by gambling and drinking. This is the nature of show business.
Peter O'Toole had a stressful career for most of his life, beginning in his early 20's as he tried to make a break in theater as an actor. Actors and actresses work in very demanding and high-stress environments. On the job stress can contribute to a variety of addictions, including gambling. Scientific research has shown that high-stress work environments can play a role in addiction and abuse.
It is true to say that Peter O'Toole's gambles resulted in the stress in his career. A bad gamble can be stressful. He did a bad gamble and that is evidenced by the outcome. I therefore believe that this is true and unless something good happens along the way, he might not be able to revive again.
There is not always an underlying cause for why people do the things they do. Yes, Mr. O'Toole gambled much of his money away. That doesn't mean that stress was to blame, or specifically his career. There are a lot of people that manage to handle their stress in more productive ways.