Sing low! Don't stress about it, sing in your range. Sinatra for example, a baritone, didn't usually sing much higher than middle C. There are techniques and exercises to expand your range if you want to do that too...rib cage expansion, muscle relaxation, stuff like that

What instrument do you play? How long have you been getting worse I can tell you honestly that my 12 TET ear has been improving. I think it's a good idea to change the instuments / timbres around often to mix things up...helps to transfer the listening skills to real world applications. The biggest ...