Hey BJCL,
I know you might be making a joke, but don't think of yourself as a "Long-Time Failure." :)
It takes time to learn, and the learning is never-ending. Even the best are always learning and progressing.
You say you "never achieved success" -- but that's such a tricky word. What is success? How do you define that? The best part about playing guitar is enjoying it! So if you continue to enjoy playing guitar every day you've already succeeded. That's how you'll want to stick with it. Find a way to feel good about it, and you'll keep going and improve over time.
Also: learning to sing and play guitar at the same time can be done (as many people have done it!). It's just one step after the other.
Have a look at this video. Camille Van Niekerk teaches the step-by-step process of how to Sing and Strum at the same time. It's just a matter of working through it slowly. It's all there!
https://www.youtube.com/watch?v=lM0JeGgTXwQ
Stay with it! I just got done with my nightly practice session and it's those tiny (almost imperceivable) bits of progress every day that add up.
-Carl.
Carl King
Director of Content
GuitarTricks
Los Angeles, CA