Hook, who was called up as a replacement on the eve of the Lions' departure for the Republic and went on to impress throughout the tour, has struggled to display the same form in an Ospreys shirt since returning to the UK.
And the 24-year-old has told the Western Mail that successive home defeats to Ulster and Leinster have left him feeling particularly despondent.
"When I came back from South Africa, I was confident," said Hook, who produced a match-winning performance for the Lions against the Cheetahs.
"I believed that even though I didn't get the Test cap, I had still had a really good tour.
"But, in this environment, things have started not to work for me. Here, even though it is regional rugby and not the Lions, there is more pressure.
Hook's comments have reportedly put French clubs Toulouse and Clermont Auvergne on red alert in the hope that his current disappointments may prompt him to look for a new challenge sooner rather than later.
The former Neath prodigy has made no comments on the speculation, however, instead choosing to focus on where he and his region can improve ahead of Friday's Magners League trip to Glasgow.
"After the Ulster game, when we were very disappointing, I went home and didn't sleep for two nights," added Hook.
"I was going over and over the game in my head, replaying situations where I didn't do things that I should have done and where I could have done better.
"The problem is that I know I am better than I have been producing, much better.
"I'm not an arrogant person, but I have confidence in my ability and I know I haven't just lost that ability overnight."