How do you know when its time? Senior dog struggles

My girl Bella is 14. She's been with me through everything - college, first job, marriage, kids.

Lately she's slowing down a lot. Arthritis makes it hard for her to get up. She sleeps most of the day. Some days she doesn't want to eat.

But she still wags her tail when she sees me. Still wants to be wherever I am. Still gives me those loving eyes.

How do you know when its time vs when they just need extra support? The vet says her quality of life is "declining but not gone." That doesn't really help me know what to do.

Anyone been through this? I know the day is coming and I'm dreading it.