My heart goes out to you, Kirsten. This is one tough decision and I know all you want is what's best for Luna. You've said the lump is between her shoulder blades. I don't know much about cat anatomy but is there a risk of the lump affecting her spine, bones, or any vital organs? The last thing I want to do is upset you more than you already are, just thought it might be a question for your vet. Prayers are being prayed for Luna and lots of
[[[[[[[HUGS]]]]]] being sent for you. As hard as this is, I'm certain that you'll make the right decision
You know your Luna better than anyone else. If you have a gut feeling about this, go with it...