Cancer/euthanization

I am feeling so stuck and I just need some advice. We have done multiple procedures to remove cancerous lumps. Each time recovery has been more and more difficult. Now that pup is about 13 years old. We have decided we would no longer operate and just keep pup comfortable. Currently has a cancerous lump has opened and growing very rapidly. It looks very gnarly. Because of the location keeping it covered is very difficult so there’s constant licking. The open surface area is Too large. I don’t think it’s even possible for skin to grow over it at this point. The growth between just a couple days has been shocking. The vet has already agreed that they would be fine with euthanizing unless we wanted to do surgery. The part that makes me feel conflicted is that pup is still relatively excited to go for a walks during the day still eating and drinking. But I can tell that sitting and laying down is getting a bit uncomfortable. How do you know when it’s time? I hate being in the position to have to make this choice. Any words of wisdom would be greatly appreciated. Sorry for the jumbled mess. My thoughts are all over the place.