I had the same experience with my 24 year old lovebird, Katie, and my 16 year old budgie, Golda. Just got weaker and weaker. Katie went in her sleep, still snuggled up to her young mate, Harley. Harley fed Katie like a baby bird for three weeks straight, even giving her mouthfuls of water. He groomed her and helped her reposition herself and was just the most loving mate you could ever want. Unfortunately, Golda fought dying with every spark of life she had. When she started labored breathing, I took her to the vet and had them OD her on anesthetic gas. Golda was a stubborn birdy from the outspoken chick she was when she arrived in our home, and she continued that personality trait right into her passing. She was the last budgie I had from the flock my first wife and I had for twenty years.
I am so sorry for your loss of a wonderful companion. Going to get Hal a new friend after? I hate just having one of a species; I feel they need one like them to feel fulfilled.