6 to 8, I really feel for you. You sound like you're in the most terrible pain right now, and I know the agony that you're going through. Losing a pet is such a crushing loss - our pets aren't just pets, they're our friends, part of the family, someone we love.
15 years is a great long life for a dog, and you sound like a wonderful devoted owner, so I am sure Lucky had a fantastic life and appreciated having someone like you.
It's so upsetting when you see your beautiful pet so ill, in pain, and unable to do the simplest things, like walk. He sounds like a really brave little guy with real spirit, giving it his best to try to walk, but if he was in incredible pain it was probably better to let him go.
Right now, I know your pain feels to great to bear. In the days after losing a pet, I've found myself permanently in tears, sobbing, and even screaming out with grief, crumbled to the floor. I'm actually crying right now, remembering those times. I felt like there was nothing to live for, if my pet wasn't here, what was the point, I didn't want to be here either. I couldn't find pleasure in anything and I was constantly tortured by my loss, desperate to sleep so I could escape into dreams where he was still alive. I know the anguish. But you will move past it. For a few days, it will be incredibly painful, but, gradually, the pain will become less raw and you'll be able to think of the good things about Lucky, rather than the sadness.
I guess you may be feeling a little resentment towards your mum about her decision to have Lucky put to sleep. She probably thought it was for the best though, and didn't want to see him suffering any more. I know I would never be able to have a pet put to sleep, but in a way I guess that could be seen as selfish, keeping an animal here because I didn't want to have to let go. I think you need to have a talk with your mum, if you can, tell her how you're feeling and let it all out, and just have a hug. It's important to have someone to comfort you at this time.
I think you would like to read 'Rainbow Bridge.' You can find it by searching on Google. It is a really beautiful and heart-warming poem, that really touched me when I first read it last year when I lost my Staffy. It might make you cry, but out of the sheer beauty of it and knowing that one day you will be reunited with Lucky, because you will. He will be looking down on you from Heaven and will be with you for the rest of your life. He'll be having a great time up there and will want you to be happy!
'Pain' from 'The Prophet' by Kahlil Gibran has also helped me through the loss of my pets. Just try to remember you gave Lucky a great life, you loved him and you continue to love him, and he loved you and still does. He's up there, happy and well, and will always be in your heart, forever. Time is a healer. You will always remember him, but in time to come, you won't think of him with sadness, but with happiness, remembering the great times, the funny things he did, the fun times you shared.
God bless you and God bless Lucky, he's in a better place now but he will always be with you. xxx