What keeps "もっと広く企業を探しておけば" so active in your mind is the private meaning you've attached to it, a story that only you know fully. What hurts in career regret is not only the missed chance. It is the feeling that your effort should have turned into a clearer life, and that disappointment cuts deeper than ambition alone. Not every regret is painful because it was huge. Some are painful because they stayed unresolved, and unresolved things keep asking for attention long after the moment ends. Sometimes the relief starts when you stop arguing with the fact that this mattered to you more than other people would guess.