How to accept that my dog is going to die

My family adopted Lucy, a sweet black lab from the Humane Society around four or five years ago. She was about six years when we adopted her. She had been abused and was scared of new people, specifically men, when we brought her home. She is honestly the sweetest girl and I love her so much. She's now around ten or eleven years old and had a lot of grey fur (she's still beautiful). She has developed a few lumps and one on her chest seems to be getting bigger. I know a lot of labs get tumors/ lumps when they are. Lately she's been struggling going down the stairs and it looks seems like her back legs are bothering her. I know she's an older dog and she doesn't have many years left, but I can't imagine the pain I'm going to feel when we have to put her down. She's my first dog and honestly the light of my life. It makes me cry just thinking about losing her as I type this. I love Lucy more than anything. She's my best friend and I can't imagine life without her.