Holding Bruiser and you in my thoughts and prayers.
I went through this same thing in December. My girl is still here, doing well, and I was more than positive that I would lose her that day. I am praying that you have the same good result I have had. They are likely to tell you that the large mass could be a blood clot and that you actually do want to hear that. Believe them. Thanks to everyone on here who had gone through the same thing, I was somewhat calmer (I am hopelessly NOT calm when my girl is in trouble). As the day wore on, the news began to sound like the surgery would work and she would come out the other side without a spleen which is a very do-able option.
Bruiser, you are so cute I could kiss you.