Moral question: Boss helped me tremendously; now I've gotten a better offer

My boss has gone through a hassle to help me find me a new position in our company. She went above and beyond with the help. After having interviewed at the new position, I got an offer which I happily replied yes to (having not signed any contract yet though). I have now gotten another, slightly better offer at another company that I interviewed at, which I want to take.

I have a close relationship with my boss as well as the other colleagues. And I feel a bit bad about it. I know you shouldn't be loyal to your employer. And I'm not. I'm loyal to my boss who went above and beyond with her help. Am I making sense? How should I tell her that I have gotten another offer at another company that I want to take?

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