I am so sorry you're in this situation. I can relate. In December 2012, we got two male budgies. Twigs (yellow and blue in my sig pic), and Pix a white recessive pied. After we had them about a year and a half or so, Pix started gaining weight rapidly and abnormally so, despite being on the same lean diet as Twigs. He blew up from 30ish grams to 54 at his heaviest. He was hanging over the perch
. We diligently gave him personal training sessions, but he didn't lose much. Long story short, two avian vets concluded that Pix had serious systemic problems due to bad genetics/bad breeding. He was living on borrowed time. I had no idea HOW soon he'd pass away. One morning I had a huge shock when I found him dead in his cage
. It was told to me that Twigs, his bonded buddy 'knew' that something was wrong and that Pix wouldn't live long. He had come to terms with it and took Pix's passing surprisingly well. I moved Twigs into the room with the parrots after that, so he wouldn't be alone (until I got Mink). When Twigs was with the parrots, he was still technically "alone". Nonetheless, he got over Pix's death right away, and they were closely bonded too. Animals do get over it... It was me who had a hard time with it.
About 10 years ago I had to have a parrot euthanized. This thread brings me back to that. It was very difficult. I had forgotten what method he used, but euthanasia is always quick. He gave me a good amount of time with her (no time can ever be enough), then asked if I'm ready. She was drowsy for a few seconds, then she was gone. It took me a good year before I didn't burst out crying without notice at random times. Take care.