How to quit a job you just started?

I work as a cashier/customer service at Dunkin Donuts currently at the busiest branch of all the stores the franchisee owns. I thought it would be a good idea since it’s busy, super fast-paced and I can work all day without getting bored. Turned out to be wrong - the hours are horrible! I have to open 4am or 5am everyday and I hate it so much. Also employees are grumpy and nobody wants to talk to anyone, it’s just not the type of environment I want to be in.

I worked at a Dunks in the mall nearby and it’s much more relaxed. I get bored easily there but it’s not as a pain in the ass as it is at the current Dunks.

I have decided that sleeping at 2am (I’m insomniac) and waking up at 3:30am is not ideal and healthy for me and would like to quit and go back to my old job (I have already scheduled a meeting with my old manager). The thing is, I feel bad that my current manager would take it personally because of the time she took to train me there.

How should I tell her? Thank you!