My dog is being put to sleep in a couple of days :(

Im 19 and we got Milly when i was 5 years old from one of my sisters friends, its one of my first ever memories and shes been with me my whole life, and a few months ago she got a tumour on her chest which cant be operated on because shes too old now.

She's getting put to sleep on thursday and im just heartbroken shes my best buddy, i dont really have any people friends but i have my 2 dogs and I love them so much they're my best friends in the whole world

My other dog is a bit younger and will be confused for a while, but how did you guys deal with losing your first dog/best friend? tomorrows her last day and we're taking her to a small stream she loved and i just want tomorrow to never end, i love her so much