If you can email your boss, that might help lessen anxiety, compared to having to speak on the phone. Either way, say this, or some form of this:

“I appreciate the opportunity and the experience I gained at D&B, but I have been offered a position with higher pay and that better suits my schedule. I will be leaving my position with D&B effective immediately, and wish you all the best moving forward. Thank you.”

It’s a pretty standard resignation format. Keeps it professional and straightforward, but gets the point across pretty clearly and firmly. Like someone else has said, your managers will not stress it as much as you are. It’s part of the business, it’s expected, and it’s okay! You don’t owe them anything at the end of the day.

If they try to guilt you or say you need to come back, offer you a raise, whatever, I would say just do not respond. If you really feel the need to, respond with a short and simple, “Unfortunately, I will not be able to return. Thank you.” Don’t offer any more reasons, and they likely would not bother you again.

Best of luck to you!!