Feeling incredibly guilty about the way I'm leaving my current employer for a new job

Hey all, I'll try to keep this short but detailed.

I received an offer for a position in IT for a small firm. I signed the letter today and forwarded it back to them. I'm really excited because this is a big step down the career path I have planned. They're even fine with me leaving the office early 2 days a week to go to my last classes for my degree.

My problem is this: the current company I work for is a nice part-time gig, and I seriously love everyone I work with and the company overall. I would like to stay on and work Sundays, so I would essentially cut my availability for every day besides that. I am currently on paid leave from said company with COVID.

I cannot for the life of me think of a way to shape a message to my bosses that don't make me look like a giant asshole for going on leave after potentially exposing other coworkers, halting the training I was going through, and then decreasing my availability to one day a week to pursue a new job.

Is there no possible way to avoid being loathed by my bosses? Should I feel as bad as I do or is there any way I could handle this situation better?

Thank you so much in advance

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