"This lack of confidence is because I know that I can't sing on key, no matter how much I tell myself "it doesn't matter, this about the words!".

No one can sing precisely on key. There will always be some variation. Singing "on key" is not as important as good vocal quality, an arresting performance based on being connected with the text and communicating with the audience.

I would suggest that the massive problem you have is an obsession with being perfect.
For gosh sakes, do you think that Louis Armstrong or Bob Dylan worried about pitch?
Or even the Beatles. There are so many examples of people who don't sing particularly well but still sing and are accepted by masses of audiences because they communicate from the heart.

Learning to sing is a relative road. There is always a way to improve but this is a noble quest. Being perfect is not. If you were to measure by oscilloscope the wavering of most voices that have been publicly accepted you would find the wavering and inconsistencies
that are prevalent. This goes for so-called "classical" singers as well.

The reason you can't sing is that you expect the impossible of yourself. Also, very few
people really like the way they sing. They may promote themselves to the public but if you asked them candidly, they would probably say the same things you do about your voice.

There are also performances by people on recording that use Antares pitch correction and this may do something to the pitch but sacrifice a "human" level of the voice.

There is another thing. You may not have yet found the right vocal teacher. This teacher should be a singer themselves and know how to produce the physical set-up to create a good sound. They also must have the ability to evaluate your personal vocal level and be able to communicate what needs to be done next to improve.

Finally, you should study acting. This will help your confidence to project in front of an audience and therefore not concentrate on pitch or production but communication.