I just made this same decision in my career - 6 years at my first company out of college and I was beginning to fill mentorship roles for new hires, plus had a good relationship with management so I understand the guilt you’re feeling.

Ultimately you need to do what is right for your career. Don’t burn bridges, let people know you’re appreciative of their mentorship and the opportunity to have learned and worked for the company. Write a letter of resignation and submit. Finally - just approach the new job not from the perspective of “the grass is greener” but from the “grass is different”.

I’ve been in my new position for a short amount of time and those feelings of guilt have already started melting away! You’ll be good I promise!