On the day Bob became Krusty's sidekick, when he'd been hit by a pie for the very first time, so many feelings had run through Bob's head at the same time:
First, there came the surprise of being hit, the dismay at having his hat knocked off, and the embarrassment of his stupid hair getting exposed, and annoyance at having banana cream splattered on his fine clothes. Then, there came anger at Krusty and those other people laughing at him, which brought back bitter memories from his school days. The anger quickly melted away when Krusty called him a genius, being replaced by a sense of pleased validation. Bob already knew he was a genius, but it was nice to have someone else call him that, even if it wasn't for the reasons he would have preferred (like his singing, his acting, or his perfect memory for Shakespeare and most other great works of literature).
Ever the showman, Bob couldn't resist bowing, and he felt more than a little triumphant when Krusty hired him on the spot, to the point where Bob didn't even notice Cecil's disappointment and sorrow for his lost dream.
That wasn't to say Bob didn't feel sorry later, not to mention his anger when Cecil irrationally accused him of accompanying him to the audition to steal the sidekick position on purpose. Bob said some things he regretted, but his pride prevented him from apologizing unless Cecil did so first. This was in spite of the knowledge that Cecil would rather listen to Florence Foster Jenkins' horrendous attempts to sing Mozart before admitting he was wrong.
But all of that was in the past, and Bob knew there was no point in feeling sad, angry or regretful about any of it. He repeated this to the point where it was practically a mantra for him.