I fail to understand how overpopulation or any other negative consequence of eternal life can be worse than death.

Death is final. Other things we can deal with.

Isn't this the reason why doctors give people with ADHD amphetamines? So that they would feel content leading their somewhat mundane lives?

Oh god, you are me! I haven't been able to identify these troubles I've been having my whole as a problem. At least now I know it's not normal and I should do something about it. Thank you very much posting this.