i know the feel, i had a dog that was with me my entire life, he was 15 year old and i was 18, so basically he was with me all of my time, because y ou dont remember the first 3 years anyway.
we had to put her down because she stopped eating and became so skinny so that it hurt her a lot when she was just laying down... it was so hard to look at
the thing was that it was the best thing to do, so if that's the same case for you then just remember it's for the best